To understand when to back, farm or gank you will have to know each role of every jungler because the times when to do these things depends on the jungle role you're playing.
Now first of all:
What jungle roles exist?
- The supportive jungler
- The carry jungler
- The AP jungler
- The bruiser jungler
- The tank jungler
- The counterjungler
Now this is just a list of what the champs are specialized in. Most of viable junglers have more than just one role but every champ excels at one of these points. I will first explain what you can apply to every role of jungling and I will later explain what specific playstyle they have.
When do I have to Farm
Always focus on Jungle farming as soon as you have your first jungle item since it gives you a huge gold boost.
- If you have no lane to gank and you need gold
- If you are Holding a lane you can lasthit
- If you are pushing a lane (alone or with the laner)
When should I go back
- If you are too low on HP or Mana to gank and/or continue jungling
- Alway if you have 1000 - 1500 gold. You shouldn't carry that much gold with you. Earlier backs on the other hand work, since jungle items are rather cheap compared to laning.
- Whenever you can afford an Item that helps in the current situation of the game. (Wards against a counterjungler for example or damage for faster clearing)
When should I gank/help out a lane?
- If the enemy is far too overextended
- If the enemy has no escape spells/flash you should camp the lane
- If you realize enemies are missing on other lanes and a lane looks like it's about to get towerdived.
- If you see a lane that needs help and where a Kill would actually really help and bring the guy that's behind back into the game (it won't help if you gank botlane for that 0/3 Caitlyn with 20 cs in 10 minutes. Farming or ganking another lane is better in situations like this).
- If you have a champion that snowballs very hard (Kassadin/Fizz mid for example) you want to get him fed.
How to play the jungle roles correctly?
The supportive jungler
The supportive jungler is a champion which usually has high cc and utility. They almost do no damage but if you gank you almost always have a guaranteed kill. Thus you should focus heavily on ganking and warding instead of farming, since you don't have the fastest cleartimes. This role isn't played that much since preseason 4. It's a Role which isn't very good but also not very bad. You could say it's a safe pick.
Champions: Maokai, Taric, Karma, Cho-Gath, Alistar
Gets countered by: Nothing
The carry jungler
The carry jungler is a champion that usually snowballs very hard. He rather depends on the CC/Help of the laners if ganking and has usually a weak early game since he has no CC at all most of the time. Once fed he can carry a game by its own. Whether you should farm or gank depends on your laners, since you need their help. You are also not that good in defending a lane but pushing works fine.
Champions: Kha'zix, Master Yi, Yasuo
Counters: AP jungler(you deal more damage than him at every stage of the game)
Gets countered by: Counterjungler (steals your farm), Bruiser jungler(you won't deal damage and he can countergank)
The AP jungler
The AP jungler is usually picked when you have an ad Mid. Your jungle speed usually is slow early but gets faster the higher your level is. This means you should gank early and farm a lot midgame. You shouldn't stop ganking at any point of the game but farming really becomes easier if you reach level 7 or higher. Usually you have 1 ranged hard cc spell and ap damage skills for the rest.
Champions: Elise, Fiddlesticks, Karthus, Fizz
Counters: Bruisers (They rarely buy MR early)
Gets countered by: Counterjungler(Steals gold/exp), Carry jungler(always deals more damage than you)
The Bruiser jungler
The bruiser Jungler is usually a champ with high mobility/damage/fast cleartime/gank potential. So overall the perfect jungler right? Well that would be too nice to be true. Unfortunately he is a beast early and midgame but lategame he's completely useless most of the time. You want to gank as often as possible to get ahead of everyone else, because once you fall behind you will be useless.
Champions: Vi, Lee Sin, Aatrox, Jarvan, Nocturne
Counters: Carry Jungler(Just countergank, invade him), Counterjunglers(They can't invade you)
Gets countered by: AP jungler (usually annoying ganks + damage),
The tank jungler
The tank jungler is just a lategame version of the support jungler. You usually have tons of CC and a rather fast cleartime. Your teamfighting ability is also pretty strong and you are absolutely unkillable. The only problem is: You only deal AOE Damage and thus you should only be farming all game unless someone really needs your help. In early you are useless but as soon as teamfights start you can turn the fight by just standing in there and AOE Bomb the enemies while you just won't die.
Champions: Amumu, Sejuani, Olaf, Nautilus, Skarner, Nasus
Gets countered by: Nothing
The counterjungler is arguably the most difficult and risky jungler of all. Really rewarding if you Can play him but almost auto lose if you can't. You are constantly in the enemy jungle and stealing their buffs, denying them farm and exp. When the enemy jungler comes to gank a lane you are there aswell and countergank and since you are a higher level than him you will almost always win the trade.
Your main objective is warding and stealing everything you can. Counterjunglers are always one of the above jungle types but they are either the fastest jungle clearers in the game or the best early duelists. You are always farming your and the enemies jungle and you only gank when the enemy jungle comes for a gank.
Champions: Shaco, Nunu, Shyvana, Udyr, Trundle
Counters: Everything if done correctly
Get's countered by: Everything if done wrong.
Now as you can see you have different priorities of when to gank or farm. For going back you can always rely on those 1.5k + means backport. Of course each champion itself also has a lot of special things to take into consiteration. But overall i'd say if you play with those rules you can learn how to jungle and you can still adjust these rules to your preferred playstyle.