Maybe once or twice a year, meaning take the bottom off and take it apart, wipe it off and clean the inside out thoroughly. Or as needed, sometimes somehow grit or sand or something will get in there and I can feel the grit as the follower goes up and down. Even with a little grit in there I've never had feeding problems though, I just think as many do that it'd be good to stay on top of this before it could cause a failure at a bad time.
Fwiw, I didn't know to clean magazines also until I'd been reading this and other internet boards. My dad never did this and didn't know better I guess and so didn't teach me to, so all I used to do was to wipe off the outside. The first time I took one of my old pistol mags apart (first time it had ever been disassembled in 25 years) about a year ago, it was REALLY nasty dirty in there. But, even like that, the magazine functioned perfectly. Disassembling and cleaning inside and out seems prudent but might not be strictly necessary.