I have T-5s on my 55gal reef tank and leave them on about 8 hours a day. When I first got the new lights, they were so cool that I originally had them on from 5:30am to about 10pm-way, way, too much! Going from a basic single 40 watt strip light to a 324 watt HO T5 setup was, well, like going from night to day. Algae issues will result from too much light, so try less rather than more.
I set my schedule so that they are on when I am home in the evening, which seems to be what most people do.