Would say "enough time" depends on the horse and the circumstances, and in some cases there isn't enough time in the sense the horse never gets back to his best. In general terms, 5-6 weeks between races seems like the sweet spot for many but obviously depends on the circumstances, how large of a top, how they have reacted to tops in the past, etc. It goes without saying that an astute trainer knows when his horse is doing well versus not but don't imagine they ever know definitively if they are going to top/pair/regress.