What exactly are "Backend languages"? I do program Java every other day, and for my private projects, I use NodeJS and Rust from time to time. My customers want PHP, but that depends on when they need changes.
I would not necessarily call any of them "backend" languages, because Java can be used in the frontend (Android apps, PC applications, browser widgets, etc.). JS is THE browser languages. Rust may be used for game-dev, embedded, WASM development, etc. Well, PHP is the only one which might be called a "backend-language", though it's in the name...
However, if you mean how often I develop non-frontend things, I'd say it's at least every other day, but rather more often than that.
PS.: You might want to tag your post with an official tag, like "General Programming", or not many people will be able to see it.