Once after every session but if I'm lazy when putting my rig away I always clean it out before the next use. Use lemon juice and baking soda when needed but hot water and a good scrubbing with cleaning brushes after each use is enough to keep me happy. I learned my lesson with my first rig(Mini Mya) which I bought H-S and it didn't come with a brush(because I bought it clearance). I rinsed it out with hot water after each session which evidently wasn't enough because when I finally got a brush set and cleand it, I was disgusted by the build up that came out of it
. If you treat your rig right it will be kind to you in return.