1 - 2 hours
My sister ran over her iPad with her car, I was amazed it wasn't completely obliterated. The front screen was shattered and the lower left and upper left sections of the aluminum back was slightly bent. Even after all that the iPad still worked in this condition.
This took a lot longer than I thought it would. The glass kept shattering into even smaller bits. Even taping the screen with clear packing tape didn't help much, except it did help with glass fragments flying all over the place. I spent around 6 hours just getting the broken panel off. Once I got the panel clear, the rest was fairly easy. It works like new.
Just take your time, there are some areas that you need to make sure you don't hit. Specially the volume rocker and wifi antenna areas. Make sure to get some compressed air, to get dust and glass particles off the lcd before it is installed. Other than that, make sure to follow the directions. They helped a lot.