I wipe out the top area, around the follower, after every range trip. About once a year I will take them apart and clean from top to bottom.
It will depend on how much you shoot.
If they are steel, it's a good idea to wipe them down with CLP once a month to prevent rust. I have heard of people putting their Glock mags in the dishwasher and having no problems at all. I have not tried it though.