It is as often as you change things. Also configuration.nix isn't just a list of packages to install and services to enable, it's also a configuration of various system components.
I'm quite new to NixOS and recently tried home manager, which basically manages your home, primarily all dot files, but goes as far as preinstalling favorite extensions on Firefox for example.
How often are people doing this, though? I get a machine set up pretty quickly (install list of packages, systemctl enable <foo bar baz>, reboot), and I'm set for several years.