What's the best time to close pick-up window?

I think it depends on your lifestyle, as all are reasonable options. My own dog sitter has younger children, so she requests that we pick up our pup by 8:30, so as not to disturb bedtime. I, however, am in grad school and allow clients to come (pickups and drop-offs) as late as 11:30pm.

As long as your window is clearly communicated to your clients prior to the stay, I think any of those hours are reasonable.

I have mine at 7 pm (8 pm June-August) for several reasons. I don't want people coming to my door after dark. I only take one dog or dog family at a time, and by 7/8 pm they have been fed and are starting to wind down. Any later than that and the owners would be waking them up, causing undue stress and excitement for the dog(s). I also like a little down time after they leave to just unwind and relax.

I see it as a case by case thing. Some regular daycare clients will finish work later and need a late pick-up or dog owners are coming home from a trip on a late flight and want their baby home with them asap so I like to be accommodating and my clients love that about me.

I have mine set for 6pm but I do have a couple repeat pups that get picked up at 7pm. I live in a very quiet patio home community and can't have dogs barking after 7pm. I am up at 5a.m. so I go to bed very early at night. That would depend on where you live, your neighbors, your city and or HOA rules, etc.