I'd probably change the tyre over and back every other month or so at best. Most of my riding is from home which means there's miles of road before I get to any mud.
N+1 is always tempting, since I could have a hard tail for the mostly road stuff and a full suspension for the mostly rough stuff, but I'm trying to be good
I may look at picking up another wheelset at some point though, which would be the best compromise.