As a kid - 10+ hours a day wasn't uncommon, 3 hours a day minimum on week days, every day the weather allowed.
As a student - as much as spare time and weather allowed, including in between classes and coming back in all sweaty.
As an adult - as much as spare time, weather and the body will allow. I start getting tired a bit earlier (say three hours in) but have also learned to adjust my skating in order to even things out if I want to keep going.
In general though, my observation is it's way more reasonable on the human body (and better for technical progression) to skate for about 90 minutes on average and space sessions out throughout the week at the ideal pace of one in every three days, as not to be a victim of what sports call 'overtraining'. But obviously nobody is ever doing that (my own self included) because a lot of skateboarding really has to do with just being out there and most especially, feels that fucking good. Just saying the 'workhorse' approach is often counterproductive, I know as much as I've been there.