i washed mine after 2 and a bit years... but it really didnt need it and i could have gone longer. let your mirror get a bit dusty it doesnt hurt it
one thing with the 8" solid tube, it comes with a nice tight fitting dust cap... which after continuos use chips the paint on the area it touches... large flakes of this paint will drift on to the primary. my sugestion is to ditch this dust cap and use a shower cap.