It really depends on the champ. Some can jungle right from the start (WW, Shaco, Fiddle are the most common), others may need help. A team (or small group of players) that downs a larger creep right of the start gets a buff, gold, and XP to put them ahead of the opponents IF they do it quickly enough to get back into lane. A full time jungler (like those mentioned above) will level off of jungle creeps, allowing you to have 2 solo lanes, and be able to gank without MIA's being called thus forcing more conservative play by you opponents.
You should stop jungling when your team needs you, either to help in a lane, with a team fight, or to push. A champ that never leaves the jungle is little better than an AFK.
Outside of champs who are primarily junglers, times I head in to the jungle are:
- Buffs. Many champs can be improved markedly by grabbing lizard or golem. just make sure others on your team don't need it more
- Leveling after a bad start. If you fall behind early, your jungle can provide some much needed gold/XP to get back into it at relatively low risk
- Clearing enemy jungle. Especially after a push/teamfight you've won. removing their access to easy gold/XP/buffs can be a big boon
- Opportunity. I will often clean out the smaller creep nests as i wander by once I can do so with little/no effort, just for the gold and XP.
And then there is Dragon and Baron. Dragon can be taken by some champs as early as the 5 minute mark (earlier if they are good or got a kill or two). Ideally, you want to get the dragon early and often as it is global gold/XP. Baron is trickier as it is late game, and can be stolen a little more easily as it generally takes longer to grab. My best bit advice is to only go after Baron if you NEED it to win. i've seen to many games where teams would have won if they pushed, but went after Baron and either had it stolen or got wiped out by the opponent right after killing it. Ward it, keep the opponent off it, but if you can win without it don't risk it. I suspect others will disagree with this, and it is likely more important at higher levels of play, but I have seen it backfire too often on teams.