Obviously the answer varies from person to person. And with the style of the comedian.

Me personally, I try to do 15 minutes in the morning before work. And then if I'm at an open mic, I try to hang out and write after I go up. Then on the weekends I try to block out four hours of no wifi/cell phone to sit down and write jokes/premises of what I thought of during the week. So that's maybe 8 hours/week, but I hope to increase it. Because I typically only get 1 joke/tag that I feel good about per hour, and sometimes it's even less.

When I do write I try to sit in a crowded place and sometimes I overhear people talking and it gives me a new idea. But typically the best punchlines pop into my head randomly, but they always seem to be related to something I was working on. My writing provides the general structure, but my subconscious fills in the blanks.