Re: how long is too long?
In her late afternoon field trip, Sophie only wants to sleep. I keep her out 1 1/2 - 2 hours and let her sleep in two or three spots: e.g., behind my head in my office chair; between my legs in the office chair if I'm still working.
In her night outing it's go, go, go. I give her an hour because I want to keep her play area time fresh and lively. She goes non-stop, but she probably has also learned that she's on the clock and should make the most of it. I would have no problem extending the night outing another half hour or so, but would want to keep play area time as 'special' in her mind.
She has a large, safe play area, and I move her wheel there during that time. Plus, she has a nice 'road course' and she has a blast screaming around the obstacles.