10 - 30 minutes
My iPhone 4s battery was basically good for 3/4 of a day as just a phone, and about 3 hours of 4G connectivity. I could literally watch the battery tick down 1% every couple minutes. I had previously taken apart the laptop I'm typing on, cleaned it and reassembled it, so I felt fairly confident about this fix.
The process took about 45 minutes. The instructions from iFixit were good and the tools were even better. I had issues with step 3 and 4 on the repair guide though.
Step 3: Use the included plastic pry tool to gently pop those clips holding the back of your 4s in place. If you simply pull on the back the clips don't let go.
Step 4: This step scared the !&&* out of me because I mis-read it. The pressure connector is NOT the black and bronze thing. That's the ground and if you remove it you're fine. Just make sure you reinstall it when you're done. I removed the ground and thought I broke my phone because I didn't read it clearly.
The pressure connector is the battery connector under the silver bracket. BE CAREFUL when disconnecting the battery. Here's what I did: I removed the battery from the glue backing and then simply slid the battery connector out. It should just slide out...don't pry it out with the tool, just slide it out.
When you insert the new battery, install the connector *first*. Line up the screw holes and you should be ok. If you seat the battery first it's harder to do. You can then gently insert the ground just underneath the orange battery screw with a pair of tweezers.
If you run into a "searching" error where your phone won't connect to your provider after re-assembly, try resetting your network settings. Settings -> general -> reset -> reset network settings. Just remember this will erase any stored network logins for wifi or vpns. I tried a hard reboot and it didn't work.
1) Read the instructions and look at the diagrams first before even buying anything. If you get nervous, find someone to help.
2) Make certain you use an egg carton or something similar to store screws as you remove them. Be *certain* you label each screw or write down what screws are where. iFixit's anti-static project tray works like a charm if you don't have anything else.
3) There's a number of parts that can get ruined by finger oils. Just buy a pair of rubber doctor's type gloves and you'll be fine. Buy some rubbing alcohol and tissues if you don't have gloves.
4) Work in a well lit area, and don't be afraid to use a magnifying glass or jeweler's eyepiece or something if needed. This is small fiddly work. The magnifying tweezers worked well enough for this, particularly with the ground and the pressure connector.
5) Take deep breaths and read, then re-read, then re-re-read the instructions. Go over the photos time and again if needed. If you rush this, you'll regret it.
I think that's it. In the end I'm pleased I figured it out and did it myself, but this is not for the faint of heart.
iPhone 4S Replacement Battery
Anti-Static Wrist Strap
Anti-Static Project Tray