In my opinion, 4 or 5 hours is enough to most of the bigger tourist attractions in Amsterdam. Maybe (more like probably) when I've been too busy enjoying the nightlife to really explore, but you can definatley see the VanGough museum/Rijksmuseum, have a walk through the red light district and go to the Anne Franke house in that amount of time. I know there's a ton of other stuff to do, but those were what I found to do and could have easily condensed it to an afternoon. bigger question though: what are the times of your layover, that will definitely effect your choices as far as things to do (night/day)