The longest I've waited on pax this year is 6 hours, but unless you have taken them far away from anywhere else, it is rare to sit like that. They usually have another place nearby for you to fly and take different pax from. Some of the long international flights (speculation here since I don't do them) probably sit a lot more to wait since the cost of flying many thousand miles empty to go back and do more and then sending another empty later to pick up is more expensive than letting it sit.