i use 4 x 25 watt run back to back they are all turned on 24/7 2 of the 25w units are over 15 years old i have never had to change the sleeves or seals on them its best to check and replace if needed the hose tails as they wear away after time
i take the unit apart and clean the glass sleeves and check everything yearly
its all about flow rate and if you match the UV to the pump or filter you are using then you wont have any problems in the past i had a 25w running on the outlet of a eheim pro 3 2080 a 25w lets you pump 1700lph and the pro 3 pumps 1700 lph
dont buy the cheap budget UV or a pond unit and you will be fine you get what you pay for
i have never understood why people turn them on and off i dont know what they think they would gain by turn it off on