I'm running a forum-based game right now; my original plan was to at least give folks a minimum once/week response, but we're keeping up with a once/day pace and the game is running relatively smoothly.
I think the majority of my players are having fun.
As far as time goes, the GM has the most work to do, though a lot of that is off-forum (adventure creation/management, etc.). I find that I'm probably putting about a half-hour to an hour into posting over the course of a day (some posts take 2-3 minutes to write, some may take 10-15 minutes to write, depending on what you're doing at that point). I don't feel it's overwhelming.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz