The books say-Every time. But then we live in the real world. Certainly anytime it gets wet-rain/snow/streams/sweat. It also depends on the cleaning product. If you are taking everything apart and using glycerine soap, that does not need to be done as frequently [but you should follow with a good conditioner]but if your using an all-in-one product like Horsemans one step,as often as possible. I do mine @ least weekly and of coarse before a show. Bits, when needed but always when putting in another horses mouth/shows. Saddle pads on sundays.
If your horse is clean your tack should stay fairly clean. Dirt degree changes, muddy days usually your girth/cinch and any leg protection. Hot summer days your browband/reins/breastplates anything in close contact or in motion to absorb the sweat.
At a show, saddle tops from ring dust
My favorite saddle is 24 yrs old and above is how it has been maintained. Hope to get another 24.