2 - 5 minutes
My iPhone 4 camera lens was scratched. I went searching for how to fix that. Saw some questionable how to videos on youtube of 12 year olds super gluing replacement lens in sideways! How refreshing to find all the professional and detailed guides on IFIXIT.com! Even though ifixit didn't sell just the camera lens I was so impressed with their guides and website in general that I went ahead a bought a new back (clear one) which contained a new lens. And while it was open I decided i should probably replace the battery too since mine was not even lasting 12 hours.
The well designed ifixit box showed up a day before schedule. I rewatched the how to video guide on ifixit. And the entire repair only took 10 to 15 minutes.
Always buy the 'with tools' item. Different parts require different tools. I bought two separate products; battery and back. I only chose 'with tools' for the back part. I should have included the tools that come with the battery because I didn't have the little plastic pry tool like the one that kept rolling away on the desk in the video. Not a biggy but would have only taken 5 minutes if i had the pry tool. End up getting the battery out with a popsicle stick - not recommended.