No, not length. Time.
I'm sure that more than a few guys think like me, tell themselves, "I paid for FS, so, by God, I'm going to get my money's worth," and fight the urge, once inside, to finish before they feel like they've gotten it. (And, to be fair and honest, some of don't want to finish too early also because doing so can deprive our partners of their own pleasure.)
But I'm sure there must come a time when we've worn out our welcome, when our partners are no longer getting and direct pleasure, or even any indirect pleasure by making sure we're having a good time.
I'm not talking about scheduling -- gotta finish because the next client is about to arrive, the kids need to be picked up at school, or she has to get her nails done. And if you've used up 55 minutes with conversation and foreplay, that, too, is another issue. This is strictly a question about the ol' in-and-out going on for too long.
Ladies (or gents, if you have something to add), generally, how long is too long?