Different yeasts work differently. With some the kraeusen seems to stay forever, with others it falls in a few days.
Just because it has dropped, doesn't mean it is done fermenting, and even when it is done fermenting (stable gravity for a few days), it is still advisable to leave it on the yeast. After fermentation is done (converting sugar to alcohol), the yeast are still working and cleaning up some of the undesirable esters they created during the ferment. Leave the beer on the original yeast for at least 2 weeks.
The longer you leave it before bottling, the less sediment you will get in the bottle, which will improve stability of the final product. You should try and leave it at least 3 weeks from initial pitch before bottling. Most of mine probably go 8 weeks before bottling and are crystal clear going into the bottle.