Sometimes once a month, sometimes more. I would go fortnightly if it was cheaper but it's quite a pricey past time now.
I will usually see the horror flicks (Saw II and Saw III) or something that will clearly be worth seeing on the big screen and lose it's impact watching on DVD (Paranomal Activity, Drag Me To Hell). It can be fun when there is a great atmosphere too. Or any film which I'm excited about and can't wait for it to come out on DVD. Particularly if it is a remake of a favourite book (which normally disappoints but I go anyway) or features a loved actor/actress and so you know you'll probably love it anyway.