You should complete a VOR test regularly, and always at least 30 days before you complete an IFR flight. An IFR flight stands for Instrument Flight Rules, and it is a series of tests that you must complete to ensure that your aircraft is suitable for takeoff before you make any journeys. Just like VOR tests, you must also complete IFR flights regularly to upkeep the maintenance of the aircraft and to have the knowledge that it is safe to fly. Every time you complete a VOR test, you must log it in your logbook.
When you complete your VOR check, remember that 4 degrees is the tolerance, so this is something that you will need to look for. Depending on the aircraft that you are using, you might need to complete more than one VOR check. This is because some vehicles come with two units, in which case you will have to check them both. It is very important to regularly complete VOR checks to ensure that everything is in good working order, and that the aircraft is in a good enough condition to fly. This is very important as if something were to go wrong with the device while you are in the air, you would struggle to fix it.