15 minuten - 1 uur
I replaced the battery on my iPhone5. Prior the old battery would randomly shut down in the 20-40% range, and charges wouldn't last more than a few hours.
Repair was fairly easy and straight forward. Took about 20 mins, and only because I had an issue snapping in the new battery. For whatever reason the pin connector just wasn't snapping into the slot until I really pressed (but be careful if plan to use force). Charges are good so far and battery is legit (be careful of the ebay versions).
When I was removing the screen, I didn't realize how easy it was for the screen to separate from the bracketing, especially with an older phone that's weathered (look at iFixit's mockup for replacing screen and you'll realize that it's in fact two pieces). Ideally you want to make sure the actual screen and frame come off in one piece. Otherwise you'll be worried about the sealant.
Be sure to take your time and to support the frame with tools when tugging on the screen. Frame can damage fairly easily.
Don't be scared though, if you are somewhat of a hobbyist, have worked with this stuff before, replacing the battery yourself can save you some good cash. Replacing it yourself is a bit more fulfilling.