Have never heard of that. There is such a thing as 'over staying' but there are no hard limits. Just one of those things that someone in the pad will probably say something about if it happens. I know I was in a pad (own beds in shared rooms) where one guy was going through a particularly bad divorce and he was there on a lot of his days off. Didn't blame the guy for not wanting to go home and everyone just kind of left it alone. Only lasted for a couple of months. Then we had another guy who treated it like a hotel room, bringing girls over to stay the night and what have you. Funny the first time it happens. A pain in the ass when you plan to get up at 0430. Definitely awkward for everyone involved. After a few of those, there was a talking to. These things tend to self-police.

All that said, your mileage will vary depending on the setup, owner,
roommates,etc.

Edit: I have heard of some pads charging reserves more vs. lineholders since you'd conceivably be around more when on call and not used.