It really depends on your operating environment: how much effort is involved in upgrading, how many systems, whether you can adequately test new releases before putting them into production, whether you’re doing anything out of the ordinary that might not work in a new release without modification, how easily you can roll back if you find something not working as expected in a new release, etc.
In general, if everything’s working OK currently and there are no security issues fixed in a particular release or new features you want to use, I’d say you could easily skip a few minor releases. In an LTS branch like 16, you could probably plan to upgrade maybe twice a year. I wouldn’t let it go longer than that though. Again, this is just a general guideline.