I prefer 6-8 hours plus a meal as a break to recharge and give space to general socializing and catching up on friendships, but I haven't done that since college really. As an adult, 4 hours is the biggest block of time it seems people can regularly commit to.
Anything less than 4 hours of actual gaming produces a pace that is just way too slow for me. Indeed, four hours produces a pace that is way too slow for me, but it can't be helped.
As far as how regular, once a week would be ideal but again I haven't done that since college. Given my adult responsibilities the best I can usually manage is every other week. I no longer have the focus to do enough game prep every week to get games to run as well as I like.