This really sucks. I left the liche quest aside for doing other things, and now when I try to go in the tower it says that Jarlaxle (the guy in the pocket plane) must have gotten tired of waiting, so I cant go in. Is there any other way? So now I can't get into Deirex's tower or Jae'Lat's tower. Am I going to miss any valuable items or loads of experience for not doing this quest?

There's no other way for you to get in to the tower, I think, unless you have a save point before then or want to cheat.

Here's what would have happened if you entered the tower (in case you're curious):

Minor Spoilers
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You would have entered, fought the lich, gotton some standard lich XP, and some goodies (the nicest of which is the crossbow of affliction +4). While going through a chest you would be transported to a place where you meet Jarlaxle, explains that he used you to kill the lich for him but then says that you're allowed to enter the house of Jae'llat where you'll encounter a good fight and some other trinkets. Personally, unless you were going through the game with the explicit purpose to play every quest possible, I wouldn't worry about missing this one.

Thanks! I dont really care that much, though I would have liked to get the crossbow, but it's too late anyway. At least I've learnt next time I get a quest which has a time limit, do it strait away :D