On the average year, nothing.
I spent $475 on a new saddle a little over a year ago because my horse was young and growing when I bought her, and she outgrew her old one. Unless her fitness level changes drastically, I don't see us needing a new one any time soon, possibly even for the rest of her life.
Accessories like bridles and girths are other things I use for the life of the horse. So long as they're cared for and your horse is an adult that's finished growing, there's no reason they should need to be replaced, unless of course you buy a new saddle and your old girth no longer fits due to a difference in billet length.
My family has pieces of tack that are 30+ years old that are still in use.
Halters and lead ropes last for nearly forever too, if your horses don't wear them on turnout. I will occasionally buy a new halter, just for fun, but I now have an unnecessarily large collection of halters that I don't need lying around. haha!
Bits are something that you might purchase a lot of after you get a new horse, until you figure out what your horse works best in. We cycled through three different bits until we settled on the one we have now. I don't think we'll ever swap this one out now. It's a 6" cyprium, lozenge, loose ring snaffle. It fits. It's legal in all competition. My horse rides very well in it, and it was hard to find, so it's here to stay.
Even saddle pads, if taken care of, and washed properly, can last for many, many years.
You'll probably buy a lot when you first buy a horse, until you find a set that fits and is comfortable for you both, but after that, you'll probably settle with what you have and not need to buy anything else for many years. Unless of course you change disciplines or your riding goals/level of competition change.