I'm relatively new to T5 lighting also ( I ran MH's on my last tank) But here's what I am running now on my 8X24w ATI Sunpower:
2 Bulbs ( ATI Blue +) are on: Noon to 10 PM
All Bulbs are on: 2PM to 9 PM ( considering cutting this to 6 hours )
My tank is 30"x30"x18"H with mainly SPS.
I think you need to take into consideration what type of Coral are in the tank. Also, what kind of fixture are you using ? T5's PAR output can vary greatly depending on what brand you are using.
HTH