i feel it depends on alot of things...
my mazda is garaged, i drive it on average about 50-100 miles a week (or less) and it has suntek hp50 tint on all of the windows sans the windshield, so really, my interior should only need a treatment once every couple months, if that. the leather...i don't mind treating that a bit more frequently 'just because'.
i can get wiped down more often due to residual dust from driving with the window cracked, etc, but actual treatment with my Optimum Protectant doesn't need to occur more often than once in a while.
just something to think about. i think it's like anything...you want to find the happy medium without creating too much work for yourself or wasting products/time.