When should I cut prospects who have started off horrible?

So my fantasy baseball plan this year was to go with prospects who were highly touted, and while some have worked out, there have been some that still have showed no signs of life. These are namely Troy Tulowitski, Josh Barfield, and Alex Gordon. Sheffield is also off to a very slow start. The question is when should I cut ties with these guys and look for people who could replace them?