5 - 30 minuten
After 2 years of heavy usage, my iPhone 4 could barely last half a day, but otherwise worked perfectly fine. I was reluctant to buy a new phone, so I thought I'll give the replacement battery on ifixit a shot.
Once the battery (+liberation kit) arrived, I set up my workspace like a doctor might prepare for a major surgery. I had the ifixit guide open like a bible in front of me, and mentally rehearsed the steps.
But.. it turned out to be pretty simple actually. I shut down the phone, flipped it upside down and unscrewed the 2 screws at the bottom (being careful not to accidentally press on the power button while it's upside down. Although I did do that and nothing went wrong, I just had to shut it down again.)
With the screws out, the back cover came off easily. Then, one more screw and comes the hardest part (for me). Slowly and carefully I pried the battery connector loose, taking care not to damage the small, black, vaguely t-shaped piece underneath. After that, pry the battery free from all the glue, place the new battery in, reverse the steps and it's basically done. Took 10 minutes.
My phone lasted a lot longer with the new battery, but still not as long as when I first bought it. Curious and somewhat disappointed, I downloaded BatteryInfoLite from Cydia and found that the max capacity wasn't 1420mAh as it should be, but 1200+. Moreover, the battery already had 20+ cycles on it. Wanting to know if this is normal, I emailed ifixit with this information. Within an hour, I had a reply saying that they've sent along a replacement battery, no questions asked. WOW, talk about great service! The new battery has arrived and it's given my phone a new lease of life.
If your only reason for considering a new phone is battery life, give this a try :) I found it easier than expected, and a lot cheaper too!