Last week, the Nemo was about 3/4 full on Tuesday but there were still plenty of seats. I was near the front of the line, and picked my spot based on being able to get out easily (plus, being able to see well), as I was solo. It’s a pretty big theater, and I can’t imagine it being full on weekdays so 15 minutes is plenty of time I’d think.
For FOTLK, it was packed full. I was early, and in the second row in the middle, but the place seemed to be completely full before showtime. I think I got there about 1/2 hour before showtime, definitely before they started letting people in.