How often are you supposed to clean tack?

What gets the dirtest?

I do my saddle and headstall maybe 1-2 times a year. But I keep my saddle inside in the house so it is not in extreme weather (hot/cold, mostly extreme heat here). But I do clean my cinch more often. Normally I give it a good cleaning after they are done shedding out for the spring, and then a good cleaning a couple times during the summer since they sweat so much. Saddle pads get cleaned about once a year, since I ride with a special pad under my thick saddle pads to help keep it dry, and keep the horse cooler. I wash my each horses bit maybe twice a year with a 3:1 ratio of water/bleach. After every ride I always spray off their support boots and bell boots, so they are clean for the next time. Brushes and grooming supllies are done twice a year and are soaked (betadine solution) and then washed (betadine scrub). Blankets and sheets are washed at the end of each season unless they have to be done for some reason during the time they are being used.

Hope this is helpful.

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.