Having replaced the screen on a 4s myself, I can say your 10 minute estimate was way off. You could replace the back glass in 2 minutes flat, but the front glass was much much more involved.
Replacing the front glass required you to disassemble every 0.something mm screw in the thing and pull all of the parts out. It took me over an hour, partially because I had to carefully document each and every screw to make sure they go back in the right place (I didn't want to swap a 0.7mm and a 0.8mm screw somewhere).
Replacing the front glass required you to disassemble every 0.something mm screw in the thing and pull all of the parts out. It took me over an hour, partially because I had to carefully document each and every screw to make sure they go back in the right place (I didn't want to swap a 0.7mm and a 0.8mm screw somewhere).