At least once every couple days...
And, generally, once a week I'll do what I call a "deep floss" where I pull off a yard of the stuff, and spend the next half hour catching up with something on TiVo while I wrap the floss completely around each tooth... one at a time (even the back ones), and floss each.
I only had to do that once to realize I needed to do it regularly.
Gum care is very important.
