Personally, I will consider my collection almost complete when my watch storage facilities look just like the last scene of "Indiana Jones" (the long warehouse shot before they fade to black and roll credits).
it all depends on what and why you are collecting. for me, i'd like to collect at least one watch featuring every movement i find interesting, both swiss and asian. in my collection, i'm also making sure to cover a number of styles - dress watches, chronographs, divers, even quartz. eventually i'd like to start collecting my favorite models of my favorite small-volume luxury brands, but those days are a long way off. in the mean time, i think 'enough' is as many watches as i wear on a regular basis or have special significance to me.