Experience is a major component of League of Legends; it allows you to level up and receive new abilities. Lane minions, jungle monsters, champions, and even wards grant you experience. So, just how much experience do you get from these things?
Experience from Enemy Champions
First off, solo killing a champion will grant 40-990 experience, the amount based off of the enemy champion's level. If you are assisting in a kill, you will receive 28-990, based off of the champion's level, equally distributed among all allies who participated in the kill. This experience is also reduced by 0/16/32/48/60%, based on the champion's level, if you are higher level than the said champion. Alternatively, this experience is increased by 16% for each level the enemy champion is higher than you.
With this information you can deduct that solo killing a champion will grant far more experience than a kill with 4 assists. This means that ganks and teamfights will not be your primary source of experience, and if this is all that is done during the game, you will start to fall behind in levels compared to your enemies who may be clearing lane minions and objectives. If you start to fall behind, you will be weaker and weaker in the teamfights that matter the most, resulting in a huge set back. You want to make sure you are going back to farm your lane or your jungle camps after a big fight or gank to catch up on any experience you may have missed out on.
Experience from Lane Minions
Killing lane minions will be your primary source of experience, unless you are the jungler of course. You must be within 1600 units of an enemy minion's death to receive experience from the minion, whether or not they have been last hit by you or another champion. Now, you can easily measure units using the champion Teemo, as they are approximately 100 units total. With this in mind, you have to be within 16 "Teemos" of a minion death to receive experience from it. Another thing to keep in mind is if you happen to last hit the minion, you will gain experience from it no matter how far away it is. This can be done by an Ezreal's ultimate ability, for example, or even a Teemo shroom.
Now for instances like bot lane, where there are almost always two champions receiving experience from the minions, there is a bit of a catch to how much experience one really gets. If there are two champions in 1600 units of a minion death, both champions will only receive about 62% of the full experience from that minion. This decreases the more champions present, with three champions receiving 41%, four champions receiving 31%, and five champions receiving 24% of the full experience from a minion death. Basically, this means that the more champions there are, the more that is "leeched" from the amount of experience that could be gained. This is why bot lane is lower level than the solo lanes, since there is almost always two champions in the lane. This also means that whenever an ally comes to rotate to your lane to gank or assist in some way, the experience gained will be spread across the champions present.
Knowing this, you do not want to stay in an ally's lane for longer than you need to be; if a gank opportunity has passed, be sure to leave the lane and return to your lane or jungle. While this technically would be giving you more experience in the long run, it is giving your ally less experience, which over time, could really hurt their lane presence. The same goes for ganking bot lane, especially since there will be three champions sharing experience instead of two. Alternatively, you do not want to be absent from a lane or from killing jungle camps for too long, as you will quickly fall behind in experience. For example, as a support, if you move up to ward and maybe assist with a kill, you will be losing all the experience from your lane every moment you are gone. This is also evident whenever you die or whenever you return to base, meaning you do not want to do either of those two things very often.
To further this explanation we can take a look at exactly how much experience is required to gain a level. At level one, in order to gain level two, you will need 280 experience, or two minion waves. From level one to level six, you will need 2400 experience, or 9 minion waves total. Keep in mind that these numbers are for a single champion and is not for shared experience.
Experience from Jungle Monsters
Jungle camps give experience just like lane minions do, but have different stipulations when it comes to how you earn it. Jungle monsters grant experience solely to the champion who last hits them, no matter who assisted in the kill or who is nearby. The same goes for Baron Nashor and the Dragon, who will only give experience to the one champion who last hit them. Now, the camps vary with how much experience they offer you, and this scales depending on the monster's level, as well.
The Raptor camp and the Wolf camp both give 95-128 experience based on the monster's level. Both the Blue Sentinel ("blue buff") and the Red Brambleback ("red buff") give 110-148 experience based on their level. The Krugs will give 175-236 experience, Gromp will give 135-182 experience, and the Rift Scuttler ("scuttle crab") will give 115-207 experience. Monster camps will only "level up" once they have been cleared and respawned for the first time. The fastest route to level three at the start of the game is if you clear both of your buffs and your Gromp, putting you at a good level to gank or invade the enemy jungle. You will want to clear your camps whenever they are up, finding a balance between helping your allies, securing objectives, and clearing your jungle. The time in which a camp will respawn can be tracked in-game, but at a glance, Raptors, Wolves, Krugs, and Gromp take two minutes to respawn, both of the buff monsters respawn in five minutes, and the Rift Scuttler respawns in two minutes and thirty seconds.
Always be sure to keep in mind the shared experience when going in for ganks, as it can lead to huge disadvantages as time goes on. For example, if you are constantly trying to gank for a lane, walking up to the minions only to watch the enemy escape, you are essentially stealing experience from your ally the longer you are in their lane. This will also keep you away from your jungle camps and objectives, so try not to be in one spot for a prolonged period of time. Another thing to keep in mind when ganking is the level difference between yourself and the enemy laner, as you do not want to walk up while the enemy is level six or above and you are still level four or five, as they could easily shut you down before your ally can even react.
Level Advantages and Disadvantages
If you find yourself falling behind in experience, you will have a harder time facing off with your lane opponent, as well as in any team fights that may occur. For example, if you find yourself at level five and the enemy team is already level six and above, they will be able to use their ultimate abilities to quickly overpower you, giving you and your team a huge disadvantage during the fight. Alternatively, if you know the enemy is not level six but you and your ally are, you can go in for an engage while they are at a lower level and without their ultimate ability. This power spike is also effective if you hit level two before your enemy, especially if your second ability will be something that can stun them or otherwise help in bursting them down.
Besides having less abilities than an enemy depending on your level, you will also have a smaller Health and Mana pool, not to mention weaker and less effective abilities overall. With all of this combined, it can lead to a very risky play that can only lead to more and more imbalanced team fights and lost objectives. You always want to make note of everyone's level before making a play, especially if there is a huge level deficit.
Experience in League of Legends is just as important as Gold or Items and is a key factor in your success. Keeping on top of your lane minions or jungle camps will balance out your level alongside your allies and your enemies, while falling behind in them will result in a less than satisfying outcome. Experience is shared between you and your allies when participating in kills or being near lane minions, so be sure to not to tip the scales and leave your teammates lacking, especially when moving in for ganks or rotating to help a lane.