15 minutes - 1 hour
My iPhone 5's battery was getting to be so erratic, the phone would power down with the battery gauge anywhere from 35-20% on bad days, or would last until 3% on good days. The charges weren't lasting very long either.
I knew it was going to be one if my easier fixes because I chose the iPhone 5 based on the iFixit teardown; the 5 was given a high repairability score because of the ease of replacing the screen and battery, unlike the 4 and 4s. The hardest part was getting the screen off because the suction cup included with my Pro Tech Toolkit did not have a ring included, so it was hard to grip.
It helps to look into the product before you buy it. Don't just buy it because it is newer or cheaper, make sure you can maintain it.