I scrub the rims with a scotch-brite or similar pad whenever I do the "big" cleaning of the mountain bike. When I get out the Simple Green and attack the gears, etc., it's usually time to get the brake residue off the rims. Sometimes it's the loss of braking power that forces me into the big cleaning. I'd say I do it every 2-3 months, depending on how much I'm riding. It makes a difference for me.