Once per week is my dream right now, and I think I'm doing okay so far.
I don't want/need(I think) to post more than that, because I don't want the art quality to suffer/look rushed. My comic is a gag-strip thing, so there's not a real story line as of yet...
If I do a comic with an actual plot, I'll be posting twice a week so the story stays fresh in people's minds.