There is a difference between deep
bedding, which is useful in your nice, dry, off-the-ground coop that has a floor of some kind and doesn't get moist, and deep
litter, which is out in the run in contact with the dirt and getting damp (though hopefully never soggy), from the rain.
With deep bedding you just keep adding more shavings/whatever on top of the messy places until you reach a point where you begin to notice an odor or the poop-to-shaving ratio seems too high on visual inspection. Then you clean it out and start over.
With deep litter you keep tossing more of the dry, brown stuff in on top of the mess, occasionally mixing it up a little (the chickens will do most of the work if you toss a handful of scratch into the litter to encourage digging), and harvest compost from the bottom layers when the pile gets inconveniently tall or you need compost.*
The only time you really
need to clean out the deep litter in the run is if you are keeping too many chickens for your square footage or if exceedingly vile weather has gotten it so soggy that you've developed an odor problem that can't be remedied by putting another 4-6" of shavings, pine straw, wood chips, leaves, etc. on top.
I let my nose tell me when the in-town chickens needed to have their coop or run tended to.
*Some people pull back the top layers of the deep litter to harvest compost from the bottom and use it directly where compost is needed. I preferred to fork out the dirtiest-looking sections and let it sit in a pile to age a bit before putting it on the garden since I was worried about burning tender plants with the hot manure.