For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
Yet there's no mandate to update if the release you're using now is doing the job. It doesn't expire.
Newer versions can always be test driven, unlicensed, to see if they have features that you need. Just retain your licensing info and the installer (though it is available on line).