Really depends on which shows are performning. To play it safe, though? I'd go with 90 minutes to 2 hours. If you are going with a friend, you can get up...stretch your legs, get something to eat, etc.
Hope you catch nice weather!
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.