Depends on how pretty i want it to be. It can be anywhere from 10 minutes to 2 hours, so that's already a factor 12. :v
Basically, when i go up to and above one hour, that's when you get things like sordsnaek and ink dragon. I don't care enough to spend more than two hours on pretty much anything, so i doubt you'll be seeing any 10 hour pieces from me any time soon. (unless you're willing to commission it for $250 :v)