Is it okay to leave a QA to rest then come back?

Use common sense.

The quests are all 'balanced' to be done in a single sitting, but sometimes due to extreme circumstances / lack of foresight that can change!

Try to avoid resting through a quest if at all possible, however, because some DM's don't like it.

I am one DM who does not like seeing parties rest in the midst of a quest. Infact, it is one of my pet peeves to see this happen. To think that you can go do half a quest, go out of the QA, make camp, rest comforatbly with no chance of the NPC monsters not comming out and killing you while you sleep is metagamming the environment.

If I am on and see this, I will reinforce the monsters within just as you have taken the time to reinforce yourselves with all new spells, or spawn monsters on you as you rest to simulate what they would likely do, or I have also made all the mosters in the area go bye bye and then placed an "escape tunnel" that they used while you were resting the 8 hours outside.

Sometimes this makes total sense, if you post guards while half the camp rests, then you rotate then rest the other half, I don't think it is meta-gaming the enviroment. Like, when the wizard runs out of spells, and the enhancements run out, the wizard might insist the 8 hours necessary to prepare a new round of spells. Also, it makes perfect sense for the enemy to try to "Escape" or build defenses while we do this. But at the same time, it might cause the enemy to become anxious and fearful. INtelligent opponents, (Like drow or Orogs) might try to ambush the party. Goblins on the other hand, might start to wail in dispair because they think the enemy is toying with them. Sometimes I think, when you rest in the middle of no where (Badlands for example) There should be a certain precentage of an enemy ambush. I am aware the Dm's frown on this, but when you are doing the Hoaran quest, and see the hole, you might send a rogue down and check it out, when he reports there are about a couple hundred undead down there, you either finish the quest seeing that you cleared the quest, go down anyway to only to gloriously retreat, (UNless the paladin hada spare bless weapon or someone has a thousand healing rods <.<) or you go out, rest at the crone, and have your clerics turn them and you smash them. (Unless a dm possesses one and tries to serve you tea :D ).

End of rant.

I prefer that people do not do this.

A) The quests are made to be done in one go.

B) It's rude to tie up a quest for 2-3 times as long as normal when others might want to try it.

Talking about the long drawn out quests, like Orog fort and goblin fort or that Lich Illithied quest that the tribe of blades sponers. But if all dm's unanimously disagree on this, I won't do unless the dm gives the okay.

Don't do it deliberately, as a general rule. If your party gets its face smashed in and you have exit to rest to rescue bodies or whatever you may do so.

All I needed to hear really, don't do it just because your buffs ran out, but its okay when operating on the rescue mission, Cool!