I clean mine lightly after every time somebody takes a bath or shower. By "lightly," I mean I take a clean towel and wipe it thoroughly, inside and along the rim, to get off any soap scum that's left behind. It's easier to keep the scum down if I wipe it while it's literally still wet, and it takes only ten seconds. I find that stuff like mildew is really preventable, if you keep the surfaces VERY DRY and the air well-ventilated. So I always wipe.
Actually, now that my kids are old enough to understand, they have started to copy this, so I wind up only wiping up after my own baths or showers.
Honestly? I don't clean it much more than that. Every once in a while when I get to feeling energetic, I'll toss some baking soda in there and scour it down. Maybe three or four times a year.
I do the tile walls and the fixtures while I'm showering-- just wipe them down with my washcloth after I'm done using it, with only water. Then wipe it down again after my shower, with a dry towel, so it's not sitting wet. And I throw the shower curtain in the wash when it gets nasty.
But I don't like disinfect it or anything. I don't really disinfect anything in my house; I like my living space to be as free as possible of nasty chemicals, and I don't think overly clean homes are healthy. I rely on ventilation and keeping the surfaces dry to discourage mold and mildew, and that's good enough for me.