For me daily because you never know when you could lose all of your stuff it could be today who knows. If it's a big site then daily,if not big then weekly because you won't lose much if it is small. Always do it before you do stuff like add themes so you won't lose things and you won't have to repay for it.
With my wordpress project I am currently working on an automatic backup is taken every week by a plugin which then also uploads the backup to my Google Drive account as a precaution to make sure I don't lose anything in case of an emergancy. When I had my forums I never really bothered with backups to be honest.
I manually take a backup once or twice a month. My server/Digital Ocean backs everything up weekly and then I have nightly backups done via cPanel as well.