1.) I have had one of them since September and the other since November. I would say that I put on an average of an hour and a half a day. (sometimes I play for a few hours and then I will go two or three day where I don't play at all). The first card probably has around 150-200 hours on it and the second has around 100-150. This is a rough estimate though
2.) They were OC'ed to 900 mhz on the core from the 810 mhz stock. That was without voltage change. I ran them at that for maybe 3 hours and then returned back to stock. Never saw temps higher than 72C on either of them. Highest temps was playing Serious Sam 3:BFE with everything maxed for a few hours.
3.) I have them in a Fractal R3 on a GIGABYTE GA-990FXA-UD7. They have one slot in between them which helps with the temps. If you have a full tower I would bet that your temps would be even better than mine.
4.) I will have to post the height later as they are still in my case. I should be getting my new EVGA Hydros today so within the next two days I will get an exact measurement. They take up three PCI slots. This thread on the overclockers forum demonstrates the size and spacing of the card nicely.
http://www.overclockers.com/forums/showthread.php?t=672877