On average, about once a month for 6-8 hours, with the same group I've been playing the past 15 years (20 years with two of the members).
We also go away for a whole weekend of roleplaying, boardgames, and bbqs at my lake house once a year in January.
My ideal schedule is once every three weeks, so I'm pretty happy we still manage to play close to that target considering how busy our lives are at this age.