20 - 30 minutes
The first symptoms started out when my iPhone would start running low on battery power quicker than normal. Then when I started using any app, the entire phone would shut down. At which point I needed to use my charger to give it a boost (even though the battery wasn't even drained). From there on everything started to decline, if I used the gps...shutdown, texting...shutdown and finally during phone calls the phone would shut down. I got fed up, researched the internet on apples site, did what they said with no resolution. I was told by friends to wait for the iPhone 5 to come out but my phone got to a point where it was completely useless and I needed a phone that worked in the meantime. So I did "alternative" searches and stumbled onto ifixit.com. What a revelation! This was an inexpensive way to repair it. Even if I messed it up during the battery exchange, it wouldn't have matter as my phone was practically inoperable anyways.
It took about 30 minutes. Kinda stressful, felt like I was performing surgery on my own kid. I would recommend watching the video once or twice first and read thru the pdf document just to familiarize yourself first before you start unscrewing anything. That cable #3 was the trickiest part, I actually forgot to re-attach it (I didn't see it) It was only after I got a call from someone and I couldn't hear the person on the other line unless I used "Speaker". So I disassembled it again, identified the problem and very carefully tried to slide the connector into place (small fingers or a tweezers would come in handy here). After it's all said and done, it worked! You feel like a super genius! A real sense of accomplishment! Now my phone works great and can hold out for the iphone5. The videos and pdfs on "How to's" were awesome!
This is a great alternative than buying a new phone. Take your time when your doing it. Make a little map/diagram of where the screws go and be gentle with the spudger when your try to lift the connectors out. The battery will take a little more force.
I'm not technically inclined, so if I can do this... ANYONE can! It's a real eye opener to the potential of what you can accomplish!
iPhone 3GS Replacement Battery
Phillips #00 Screwdriver
SIM Card Eject Tool