Here is a helpful link on bathing.
http://birds.about.com/od/grooming/a/bathing.htm
In regards to how often? Best practice is to provide a water bowl away from perches (so no droppings get in etc) as what a parrot needs, and what a parrot wants can be two very different things. This way they can groom etc as they please in whatever manner they wish.
If you just cant do that, then best idea is to have your indoor bird get a spray or few during the warmest part of the day, or if your home is constantly quite nice, then whenever you find they like it most... trial and error comes into play in that regard.
There is no guide for times... or how often. But birds, like cats, constantly groom themselves, and the more you can facilitate this the better. A birds health even if its not flying around alot, is heavily reflected, and impacted, by their ability to maintain a healthy feathering.
In short, if you have the time to do it daily, awsome, as sometimes its also just loads of fun for the bird. If not, every few days is a good idea. Its not just the feathers that require this moisture, its their nasal passages as well.