Not sure about SS but at SA they didnt tell us a time limit. We took one out for probably 2 hours one day. We made sure to keep an eye on the boats and as long as there was one available we didnt bother to head back. They were not used as much as I have seen at some Mexican resorts.
No time limit just have to stay in a certain area and be back by the time water sports closes as far as I know. They never told me a time limit and I checked out the hobie a few times while there in Aug. Just make sure others have time to use if it seems like there are people waiting. also during a class I think they have to all be back in.
Just back from CSS. Husband took out the Hobies. Never a line to use them at CSS, and no one seems to care how long you have them out. I am sure if you were out a real long time, and people were asking to use them and they were all out, they may call you back in. No one is watching a clock !
I can't speak for the resorts, but I would say a rule of thumb would be how many people are waiting to take them out. I've seen many days when they just sit on the beach with no takers. On a day like that I don't think they care if you keep one out most of the day. If they have 20 people on a Saturday waiting for "their turn", the time may be a whole lot shorter.
I think their main concern is to keep an eye on you to make sure you stay safe and don't encounter difficulties. Other than that they do not care how long you are out as long as you're enjoying yourself! Out of respect though, I would not stay out if it is getting close to the end of the day and quitting time for those guys.
It only takes me about 1.5 hours before I think its time to head in and hit the bar anyway.