.. go there when you feel like it? I'm into Ch.7 and I don't want to leave BG for an extended period of time, in case Sarevok can win by default. Spoil if necessary.

Believe it or not both your options are correct. You can go whenever you choose, but there is a quest tied in with the place. You get it from a guy in Ulgoth's Beard. Which you can get to any time once you have access to Baldur's Gate, though for some reason every one else claims they can get to only after they beat Saervok. Perhaps it has to do with the BGT mod I can't remember.

Ulgoth's Beard is available at any time in normal BG1.
In BGT (a BG2 mod) you can only go there after killing Sarevok.

oh and there is no time period for doing the game that if u dont do it in a certain time Sarevok wont win, so dont worry,take ur time and make the most of the game!

Thanks guys, that's what I wanted to know, if I could go do other things and come back later.