Most quests do not have a time limit. IIRC the game will let you know explicitly if there is a time limite involved. Quests with time limits are (minor spoilers ahead!):
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- The one where Jaheira gets cursed by Baron Ployer. You have a few days to undo the curse, or Jaheira will die (and cannot be resurrected).
- The quests you do for the matron mother and her protege (can't remember her name) in Ust Natha. If this makes no sense to you, don't worry, it means you haven't reached that part of the game yet.
[opinion]The only obvious weak point in the overall plot of SoA is that despite Imoen being jailed, and the character's determination to save her, there is no urgency to it. You can take months doing all the side quests and there's nothing to indicate that Imoen is languishing somewhere in a dank, gloomy cell, with unspeakable acts of torture being done to her.[/opinion]