Permanently prune backups created by Time Machine
I am cleaning up old Time Machine backups, which slows stuff down to a grind. I want to tell Time Machine to delete all backups older than 3 months. I'd also like for Time Machine to keep that as a default rather than having to do that by hand myself each time. How do I do that?
I was able to find this app, but I've never used it, I don't know if it works: Time Machine Editor
It purports to edit the frequency rules .plist for you.
Otherwise:
These are the rules. If you don't like them, use a different backup solution.