Supposedly you can harass the enemy jungle a little by killing the valuable creeps in a jungle pack and leaving one of the cheap minions around to delay a full respawn. How does this work exactly? If I see a solo minion up at one of our buffs, should I kill it or let it be? Is there a difference between leaving two minions up and coming back later and killing another to delay a buff/respawn?
A given camp will not respawn unless ALL of it's creeps in that camp is first dead. Only then will the respawn timer start up.
If it's a camp on your side of the jungle that has only one creep left, make sure to kill that creep so that camp actually respawns.
A great way to counter jungle is to steal all of the creeps except for one of the small creeps per camp. The enemy jungle will have to kill that creep and wait the full respawn time for the camp to come back.
I noticed that the question is not very specific so I split the question and put the answers for each question inside of it:
The mechanic of camps are: they will respawn only after all minions from that camp are dead. the counter start by the time the last minion die.
You should clear it. Unless the camp is blue or red and you believe that none of your team (including you) will get in the respawn time.
NO, "A monster camp will not respawn unless all of its creeps have been slain. For example, if the Lizard Elder has been slain, but his two smaller lizard companions have not, the Lizard Elder will not respawn until they have been slain. Jungle invaders may deliberately do this to deprive their opponent of jungle gp/exp."
Today (03-09-2012) respawn timers: