I've only been into keebs for a few years now so I have just a few sets of keycaps and boards, but so far, I'd say every 6 months or so (or once or twice a year) is my pattern of changing things up. I'll be building a new board toward the end of the year so that'll give me something else to build, and I already have my eye on two new keycap sets, so I think when I have more options, I might change out my main board every 3-4 months or once per season. That's mostly for keycaps, but usually if I'm changing out the keyboard, I'm also doing other changes to my whole desk setup, so I just like to make things match.

I don't know that I'd change switches that much though since I like to leave the switches as they are unless I find something better.