How often should you have your septic tank pumped out? It depends on the size, the people in the house, and whether you have a garbage disposal. (There's a chart with this article from the HouseMaster newsletter.) There are variables, of course.
Link: HouseMaster eNewsletter
We had our septic tank pumped out again recently. Those new filters can be a problem, btw... Our septic system (tanks and drain field) is so big that we call the whole area The Mesa. When they were working on it last, they asked if we'd be using a garbage disposal 'cause if we were, they'd have to make it even bigger.