We pretty much have our DLP set on at least 6-8 hours a day. On the weekends it has been known to be on all day. I know we shouldn't do it but a lot of times it is on while no one is even watching it. I like leaving it on for background noise while I'm on the computer. Yea I know, dumb.
Anyways we got the tv last Novemeber so I figure that's about 330 days ago. I'll be conservative and say for those 330 days it was on average turned on for 7 hours each day. That's 2310 hours. I know the bulb won't last a real long time doing what we keep doing but that's ok, we bought the tv to enjoy how we want. So far the tv has worked like a champ and the brightness hasn't diminished at all where we would have to go into the menu and increase it's setting.
Now keep in mind I didn't leave it at the torch setting like they are usually set at in stores to make them look brighter and supposedly better in those well lit stores. I have mine set on natural and the warm color temp which is significantly lower contrast and brightness than the brilliant and cool color setting. This is not only necessary for the best picture quality, it also helps prolong the bulb life a lot I would imagine.
I'll post on here when I have to replace the bulb, hopefully it's a long time from now but if not the warranty will cover it for the next 2 years so no more extra money out of my pocket. And like someone said, once the bulb is replaced (by myself) it's like a brand new tv right out of the box again. I prefer this method personally rather than having to deal with anything like dead pixels on an LCD which would be costlier than a bulb replacement when out of warranty I imagine and would not be able to be repaired by the average person at home.
Not saying that it's common to have dead pixels or anything but the potential is there.