Run it as little or as much as you want - some just run it for a day or two at a time when they feel they need it - others runs theirs 24/7 (and of course replace the bulbs much more often).
I use mine on a regular basis, for weeks at a time, and find that it helps to keep my tank water crystal clear 100% of the time - this of course along with frequent wcs, strong filtration, a rigorous media cleansing routine, and the use of Purigen and filter floss, as well as the use of fine pre-filter sponges on the filter intake tubes.
Not to mention the job a UV does in ridding your tank of any floating algae in the water column and harmful pathogens.