I guess I wasn't clear. The post was already getting overly long. There is certainly nothing wrong with setting an arbitrary session length and sticking to it. This is done by many pros and experienced recreational players, (and recommended in BJA), and I too highly recommend it.

With modern surveillance techniques, "camping out" at a table is no longer really an option (indeed, if it ever was). Give them enough time to look at you and they will figure out what you are up to. Everywhere, one reads, "Keep your sessions short." This is sort of a given in my post, and I should have stated so explicitly.

My post should be taken in the context of reasons for deviating from this predetermined session limit. When is it a good idea to leave a game before your session time limit is up??