By Isaac Butler
If your show is longer than 80-90 minutes, please insert an intermission. If your show has an intermission and isn't written by a genius, please make sure that intermission comes before the 80 minute mark. Theater takes more energy to watch than film, and stretching your show beyond that point risks taking something delightful and making it exhausting.
There is no shame in having two forty-five minute acts. If you think your work must be seen in one sitting, than take this time limit as an opportunity to edit. Concision is good for most pieces of writing and productions; it forces you to compress energy and meaning into each minute. I can't remember a 100 minute show that I thought wouldn't've been better at 90 or even 80 minutes in length, nor can I recall a two hour sans-intermission show that actually had enough worthy content for its running time.
Please know: I love messy, overstuffed works (I have particular affection for Kushner's Hydriotaphia and, you know, Shakesepare) but a little bt of consideration for your audience is not a bad thing.
If you happen to craft a work in which the audience is free to come and go as they please, then you can do whatever you want.