I submit it once a week as I add new content to my site constantly. The only thing that bugs me is that after all of the times i have submitted, they still can not fix that one 404 error i get. It really annoys me
I use a blogspot blog. In the Google webmaster tools i use my RSS feed as a sitemap, thus whenever i post a new entry, it is automatically submitted to the sitemap.
Ironically the sites I run where I don't submit often (once every few months) do much better in the G Serps than the ones I do submit frequently (once every few weeks)
I never submit at all. I use google-sitemap for its other functions, like search-queries and stuff like that. For a static-HTML type website, that has good internal structure, submitting a sitemap is not necessary.