Best advice from all the "experts"
is to do a complete backup at least once a month......but then who listens to the experts.
If you make any major changes (you determine) then make a backup so you don't have to do them again. I am lucky if I make a full backup every 6-8 months, but I also am aware I may have to do a lot of reloading if I crash so I make sure I have all my program installation files in a safe place.
At work we don't even back up individual computers except servers. We have a CD (Bootable) that will reload everything we need to if a computer crashes. If the individual doesn't save there work to a server....they lose.