How long is long enough to play?

My DD often goes down to the neighbor's to play (ages 5 and 6) and vice versa. This is perfectly OK with both us parents. However when their DD is here she usually bounces back and forth from home periodically whereas my DD would stay all night if no one told her otherwise. Time and again the mom said it's fine with her because they fend for themselves but still I can't help but wonder if I should be checking periodically, say after 1 hour? 2 hours? At 3 hours should I insist she come home even if the mom still isn't bothered by her being there? IDK, I just don't want to get annoying even though she has made it clear DD is not.