I have my 11w unit on 24/7. The guy from Auburn LFS tell me that, if you switch on and off too often. It will shorten it's bulb's life...
And.....don't even bother to clean inside...I tried...and all most beark it..and after I opened it. Inside the tube is as clean as new (nothing can servive around UV bulb)...wasted all my effort..
I currently run mine 24/7 and hsvehad no trouble with eitehr of my two tanks running them. They are both the pro aqua ones and am more than happy with them. I know George at Slippery Little Suckers is currently stocking some of these.