I think its generally ok to leave it in for up to 24 hours. The only reason they say not to leave it in for long periods of time with tampons is because of the risk of TSS and if you don't have that with tampons then there is not really a risk of anything is there?..
I have a friend as well who regularly leaves it in for long periods of time (I think 24 hours) because she is on the pill or some kind of medication which means she spots a lot, and she loves the mooncup because she can just leave it up there and not worry about anything.