Multiple times a year. If your system is well designed it should be trivial to move between different versions. As languages evolve they tend to get faster and fix more bugs. Your tests should catch any bugs if the new version breaks anything.

If you have performance concerns you can update a single server and see how it performs relative to the others. Once you're comfortable that everything is working well you can gradually migrate more servers until everything is running the new version.

Each service should have its language version pinned instead of relying on a single global version. That way you can upgrade projects incrementally.

If you're really worried you can keep both versions of the language installed at the same time so you can quickly revert in case something goes wrong.