I do not like others to accompany when shooting as many times I will just sit in the environment, listening and looking before I start. How long is too long? If you come up with an answer then you probably haven't spent long enough. I usually stop when my feet hurt, or freezing, or no more film. How long do you wait for subject matter to evolve in one place? Sometimes it never happens. If accompanying me please bring a book "War and Peace" would be appropriate, and please don't talk to me, or ask to look at the ground glass and then say "Hey, it's upside down"