i think it depends on how much content is being released with each update. if we are talking like...1 page/5-ish panels at a time, i would expect updates once a week or once every 2 weeks at the longest. i think id want web comics to be like tv shows. come in seasons and update weekly.
The schedule should be consistent for the sake of keeping your audience's attention. Even if it has to be every two weeks or so, if the schedule is relatively consistent, the audience will continue to check the comic. You can sacrifice quality a little bit for a quicker schedule, as long as you go back and refine the artwork in a timely manner.
For example: sometimes Coelasquid (Creator of Manly Guys Doing Manly Things) will upload a page that is junk inks or just flats, but she quickly goes back and updates the artwork to completion, all while maintaining a consistent schedule.
Well in my experience, I prefer webcomics that are easily accessible and have a time schedule. Ava's Demon is a great Tumblr webcomic with excellent art but updates so infrequently, I would forget about it had I not been watching it on Tumblr.