5 - 30 minutos
I ordered 2 iPhone 4 replacements on eBay (for like $8 each) and neither of them worked.
I replaced the front display in the same repair and both went swimmingly!
The battery I got from ifixit had zero charge when I installed it - make sure to charge up the battery all the way before testing the phone, especially if other repairs are involved. NOTE: Make sure you tighten the standoff screw on the logic board all the way down (but not too tight) or else the battery screw won't go all the way in and your phone will do WEIRD THINGS!