I tend to run Ubuntu LTS in my systems, and only upgrade between them every 2-4 years to a next LTS or the one after (Ubuntu does Long Term Support releases every 2 years and supports them for 5 years). I will reformat once in a few upgrades, so on a scale of 6-10 years.

Now, I run a bunch of lxd containers for development, and they are a bit more ephemeral and might get re-setup more frequently. This allows me to avoid having a base system with dependencies for stuff I don't really need.

I sometimes also reformat when I introduce new disks or replace them with bigger ones.