Well, I agree it's possible they'll change their routine with repetition, my gliders have changed up their routine (mostly on their own) since I first homed them, and definitely a bit seasonally.
Any time from evening to early morning is a good time for out of cage play. When they're up and the most active would be the ideal time, but that doesn't always fit in with routines we have to be on. Better to work off their schedule a bit then to skip out of cage play all together.
I personally like morning playtime best, but not 4am, lol. I loved when my schedule and living arrangement allowed me to get them out anywhere between 6-6:30 am till about 7 am or so. I would do this three times a week and found clean up easier (less pooing and peeing, lol) and also they were more willing to go back into their cage when done.
I've had a lot of disruption in routine and living arrangements since 2014, and have struggled getting a good routine to stick with. So I just do the best I can, and it seems evenings are the most convenient all around with my latest work schedule.