I don't make any effort at keeping track of the last time I cleaned my 22RF bbls - just wipe 'em out with a couple of patches saturated with Ed's Red solvent, then one or two dry patches, after every range session. Most of my barrels are hand-lapped customs - Bartlein, Krieger, Lilja, Shilen - and after putting a hundred or so rounds of std vel match ammo through them, they very, very seldom have any lead fouling. It's just so simple, easy, & quick to use a rod guide and wipe them out that I can't see any reason not to.