5 - 30 minutes
I've been putting off replacing my iPhone 4 because I hear the new iPhone 6SuperExtra will be made of a ridiculously-durable aluminum/unobtainium alloy that they use on spaceships and stuff. But the battery has gotten so worn out that it shuts off more often than Rufio, my narcoleptic cat.
Dude. It was splendid. The charming girl in the how-to video answered all of my questions and her warnings were dead on - my iPhone had lots of adhesive but I was eventually able to pop the battery with a bit of prying around the edges. Besides that, it was a textbook replacement. Now the battery lasts all day and still has juice left over when I go to sleep! I felt like Danny the Champion of the World afterwards, so I treated myself to some cotton candy and a sauna visit. Not simultaneously, though.
Wear sunscreen. Watch the how-to video a couple of times. Have it open and paused during the repair so you can refer to it if needed. You'll also want Rick Astley open in another tab in case you run low on confidence and need a quick pick-me-up in the middle. Always pay your bills on time, be nice to your friends, call your parents, and never drop your phone in the toilet. And trust me on the sunscreen.