A license lasts permanently. All registered users can receive software updates of the same version they purchased free of charge. Updates are minor feature additions, enhancements and fixes, within the same version number, i.e. 10.0, 10.1, 10.2 etc.
For instance: If you obtain Navicat 11.1, you will only entitle to free upgrade to Navicat 11.1.1 or 11.2. For upgrading to Navicat 12 or above, you will have to pay for it.