I shoot for once a week during the warmer months. It doesn't always happen though. When it becomes too cold to do it myself, I take it to the car wash as it needs it, typically each time the roads dry out after a storm. It's next to impossible to keep a clean car during the winter around here, though.
Also, I still need to wash the minivan after my UP trip last week. For those not familiar with Michigan's Upper Peninsula, it's absolute bug paradise up there. The front end looks HORRIBLE right now.