Some spoilers;

Very early on, I met Nalia and followed her to her father's castle:

http://forgottenrealms.wikia.com/wiki/Nalia_de%27Arnise

I find there I'm probably underpowered - mages cast Confusion which affect my whole party and I don't yet have the Cleric spell to counter them. Does anyone know can I simply leave the castle and return at a later date to finish the quest?

P.

Yes; very few quests in BG2 have an actual time limit.


That said, you should be able to clear out the d'Arnise Keep at the start of the game. The Umber Hulks with their confusion are somewhat nasty, but they're definitely beatable with an early-game party. Have you tried varying your tactics or spells a little? A few summoned creatures as a distraction might work wonders.

No you can't leave if you want to complete it -- there is no way back to the destroyed keep. It's a time-sensitive quest -- you're trying to save her family and estate! It is a quest that is designed to be taken early, so you simply have to work out a way to deal with the Yuanti mages -- it really isn't that difficult, so long as you don't adapt Rambo tactics.

try skeletons from your priest spells, they are inmune to confusion, OR invisibility on a meele character for pick by surprise the yuan ti mages

If it's the Yuanti mages that are causing you grief, use skeletons as suggested, and get your mage to dispel the protections that the enemy mage puts up on itself -- it will keep it busy trying to put them back up -- then use ranged attacks.

If it's the Umber Hulks that are causing you grief, a well placed Cloudkill will see them off in seconds -- they are extremely fragile to Cloud Kill. I usually have Nalia quaff a potion of invisibility and use the wand that you found in Chateaux Irenicus. Or, alternatively, use the dog meat stew that one of the servants told you about.

Thanks for the advice. Just to note that I tried returning to the city, waiting 40 hours, then returning to the keep. I found I could re-enter fine. So not sure this is a time-limited quest.

P.

Oh... ok. Was it still the troll-infested ruined keep?

Yes the only quest I have ever failed because of a quest timer was the Ranger Stronghold quest.
One of them has a very short time to complete, you virtually have to do it as soon as you are told about it.

Most of the quest timers are just there to advance the plot.

@OP: The game is going to get a LOT harder.
This is one of the easiest quests in the game.

There are a ton of hints about how you can avoid combat with the umber hulks.

Also, confusion =! loss. A confused character has a high chance to keep on fighting an enemy.

I'll try to give the time-based quests here (the other quests are NOT time based or give you a specific time limit, like "meet me at ... in 2 days"):

De'Arnise Keep, if you have Nalia in your party. You may either join her temporary or dismiss her straight away, this way the quest will not be timed.
If you have a save before going to keep, try this. She'll give you about a week before becoming impatient and striking off on her own, then do what you want, when your ready finish this quest by meeting her at the keep.
As for the umber hulks, try this:
Have your thief open the door with your party out of sight. Then have him hide in shadows behind the door, ready to close it. Then have your mage cast a cloudkill into the center of the room and run back out of line of sight. Finally, have the thief close the door. Wait a minute..... and you have dead umber hulks.
For the mages, try sending one summoned monster ahead by itself, you don't have to see where you are sending it, then HiS with your thief, sneak into place for a good backstab, charge in with the others. If you can get mages to waste their spells before fighting them they are real pushovers, especially if you "cow tip" them from the shadows!

Umar Hills quest, if you have Minsc in your party.

Rescue Garen's child from Firkraag, if you have Anomen in your party.

The Druid Grove, if you have Cernd in your party.

The Book of Kaza, if you have Korgan in your party.

The Nether scroll, if you have Edwin.

Obtaining the Planar Jewel, if you haven't even joined Haer'dalis but immediately send him to the Five Flaggons after rescuing him.
In other words, rescue Haer'dalis, then help the actors, then you can do Bard's Stronghold quests.

All of the NPC's quests.

All of the described quests have 10 days limit, starting from the time you took the quest. For example, if you take Firkraag's quest and after 15 days join Anomen, in a couple of minutes he will spit out several lines of disapproval and will leave almost immediately. This won't happen if you fulfill Firkraag's quest before joining Anomen.

The Ust-Natha quests are timed. All of them.