If I were seated without a reservation, and told at that time that the table was reserved for another party beginning in, say, an hour or an hour and a half, AND if the service/kitchen went off without a hitch (no major delays getting the waiter to take my order, food cooked properly and in a timely manner), I would have no problem with keeping an eye on the time to make certain I did not inconvenience people with a prior claim.
OTOH... I know how to rent a table. I can sit down at a four top and look at a menu and know how long it should take for the average diners to roll the table to the next group, and know what their average bill will be - and tip accordingly if I overstay the average. If I'm there twice as long as the average diner, probably for half that time I'm only wanting bread and tea or drinks, while I chat. That's a good bit less work for the server, but I'm going to tip for the time spent AND on my ticket amount. If my four top ate an average amount, and our bill came to (let's low-ball it) the average $100, and I'm having enough fun that I want to stay - sure, I'll tip the 20% on the bill, and another 20% for the double stay, and probably another 10% for worrying the server all that time, if the service was good and they were cheerful about it.
If they're pressuring me to leave and there's no one waiting? Oh, that could get nasty.
***Note: Not if there's a line. If there are people waiting, I'll eat and clear out - that's not a fun chat environment, anyway. UNLESS I had had a reservation and warned the restaurant ahead that we would be a long party - and that's restaurant planning issues again.