20 - 30 minutes
My Iphone accidentaly dropped inside a glass of lemonade (fail I know) and as a result it turned off when disconected from the charger and needed a replacement battery.
It took me between half an hour and 45 minutes because I wanted to be carefull not to mess anything up because of my inexperience.
The screws on the inside of the Iphone 3gs have different sizes although they have the same head. You should pay attention to the place from where you take each screw. Also for the unexperienced you should read the instruction guide before watching the video on Ifixt, because the video doesn't go into details with every step and if you miss a step you can break something.