How early do you arrive to see a play?

I'm planning on taking my friend to see Mamma Mia at the Fox Theatre for her birthday. How early do people usually get there before the show actually starts? Does it matter if we only get there 15 minutes before showtime? I'm taking the ACT that day and it only finishes two hours before showtime, so we'll have to really plan our time wisely as to not be late.