How often should a hoster upgrade to cPanel

I'm guessing you're referring to the theme itself. That doesn't change that often, honestly.
cPanel has used x3 for years. Prior to that, it was X, can't say before that.
Recently (as in the last year and a half or so), the push has been for paper lantern, as x3 is going to be retired after so long in the field.

cPanel itself updates by itself (if the host is smart) regularly. It's just the theme that doesn't do much updating, which is ok

 

I wouldn't say 'never', but yeah, it rarely changes. You wouldn't notice that often