Raptor, first I'd figure out how much time you plan to have the concert for. If it's 2 hours, you might want to go for 4 players at 30 minutes a piece, planning to run over by about 20 minutes (or 5 minutes per performer). As a general rule of thumb, over the years, I've found if you tell a performer 20 minutes, they'll take 25, etc.

This is assuming they are all equal in "draw" power. Now, if you have one act who is REALLY a major act, allot that one say twice as much as any other. For instance, one Major act, use them for 1 hour, and 3 others at 20 minute sets.