I usually like to book a minimum of 2 hours (and I like late punts), so that I don't have to think about the time, with resting and chatting in between (usually 2 pops nowadays, the younger years of multi-popping have gone). Also, I enjoy foreplay more now that I've gone into the grey years, gives me more time to ramp up to the better bit of the punt. And for some reason, apparently some WGs even seem to like it. Very odd...
And, because 2 hours is plenty of time, I easily bring up the time I need to leave and actually ask them, is it time? Some say, yes, some say "we still have time for some more fun", etc..., never been booted or called to attention, especially in the latter years.
However, there's a downside to longer punts. If you are not enjoying it, it's a path to purgatory...
On a side note, I have had the luck of, in a couple of really late punts (midnight or later), the SPs asking me if I wanted to stay over some more time, off the clock, but I believe that was just because we may have accidentally clicked and, they were certainly completely independent (one on her own flat and the other in a hotel room) and had the next day off. But that was twice in a lifetime, don't believe will ever happen again, though I wish it would...