A UV should be run 24 hours per day 7 days per week
The tubes are rated to run for 6 months run 24/7
Parasites and green don't sleep so it needs to be run 24/7
Anyone who tells you any different doesn't know what they are talking about
If the tube is turned off even for a day or 2 slim starts to build up on the quartz sleeve blocking out the UV light
Turn it on and off also puts strain on the tube and may cause it to blow before its 6 month time scale