Every few days on the average, although I might go out a couple times some days, then not buy anything for a week.
There's a Safeway 1 block from my house, and obviously I go there for last-minute things, but I try to avoid them. I shop at five other places regularly. Grocery Outlet, PCC (Puget Consubers' Coop), and QFC, which used to be a good, mainstream supermarket, but now Kroger owns it and things have gone downhill. Also there are the two produce stands - McPherson's, which has poor to OK produce, but terrific prices, and uh, the Filipino place, which has not only produce, but canned goods, meats, and well, a lot of things I just can't even imagine without going there.
Oh, yeah, there's the Columbia City Farmers' Market too, but it seems I never have the day off when they're open. Too bad!