I'd agree with GD on that, perhaps a little less sometimes. Off course what is being talked about here is the initial effects (the trip). If you go about it the right way, the "effects" can be yours for life. Do plenty of research into "set and setting", and have a good old think about why you want to do them.
On me about 3 to 4 hours...then I get really tired and have real wild dreams...I can't sleep for a couple of hours then my mind finally does rest and I could sleep...and even after the effects of that day the next day I still feel a bit paranoid..like someones following me..and seeing shadows and shit...I stay like that for a couple of days after...but then it goes away...it all mental really...