I'll tell you:
With the set off, QUICKLY press Mute -> 1 -> 8 -> 2 -> Power.
When I say quickly, I mean quickly. If you do it correctly, you'll see the service menu on the screen. Since I haven't tampered in there for a while, I can't remember which menu option shows the number of hours used, but it's definitely in there. Use the menu and enter buttons to navigate.