This has been a problem for me also, especially when I first started on Rover. I was more willing to be flexible, as I really needed the clients and the reviews. Over time, though, clients would abuse that more and more. "Give an inch and they take a mile" type of thing. Now I have a system in place, and it's made my bookings a lot smoother.
I let them know my pick up and drop off times at the M&G. Unless it's a regular, consistent client that I know is always awesome, I don't budge from that. For me. I do not allow drop-offs for boarding before 7:00 AM. I do not allow pick-ups for boarding to be done after 10:00 PM.
If you are more than 30 minutes late, we will need to reschedule. I also have walking clients during the day. So I let them know at the meet and greet that if they think they are running late, give me a call. If they're running 30 minutes late, we will need to reschedule the drop-off according to my other clients. I always let them know upfront that I try to keep my schedule full, but flexible. Even still, I won't make another client wait because they're running late.
Clients will ask"If I pay you extra, can I drop-off/pick-up outside of that window?"...
If they need to drop-off their dog before 7:00 AM, I ask them to add one day to the booking and drop-off the night before. Mornings are hectic, and most people that want to drop their dog off before 7am typically end up running late that morning anyway.
If they want to pick-up their dog after 10:00 PM, they may pay for an additional fee for "after-hours" care. Generally half of my boarding rate.
Even if you're firm with your time windows, people will still inevitably be late. 5-10 minutes, ok. Not a big deal. We're all human. I'll wait 15 minutes before calling the client. 30 minutes late...? See #2. I think the important thing, and the thing I learned the hard way, is to let the clients know at the meet and greet (or at least before the booking is confirmed) what your boundaries are. Put your boundaries in your profile and reiterate them when meeting new clients. This way, when you stick to your rules, they can't say they didn't know.