It depends on why you have a UV unit... if you're trying to treat a freefloating pathogen (rare), you should leave the UV on 24/7. If you're trying to treat green water, you only need to leave it on until you've cured your problem. You can use it again if the green water reappears.
For most FW setups, UV isn't necessary and won't yield many benefits. The best case for them that I've heard is for people with green water (unicellular algae blooms) that hasn't responded to more traditional attempts to control it.
Jim