How often do you reformat your personal computer?

Personally in the past i often formatted my laptop every two months or so because i often experimented with the operating system and broke something or just trying out a new setup. Currently i haven’t formatted for almost year because 1: I don’t see the need right now. 2: I don’t have enough backup space to backup everything that is on my machine. What is your reason for reformatting (or not) your personal machine?