20 - 30 minuten
My 3GS (inherited from my wife) - had a pampered, lazy existence, but after 3 years the battery was becoming very tired. I'd taken over using the 3GS as its network reception was better than that of the candy-bar feature phone I'd been using, but battery life on idle was less than 1/2 a day. I wasn't about to toss a smartphone in immaculate, as-new condition, and I wasn't about to stuff about with a suspect battery kit of unknown provenance from EBAY. So the battery and toolkit from ifixit seemed the best way to go (and a lot cheaper than a new phone). Besides, I'm allergic to throwing out perfectly serviceable kit, all for the want of a battery.
As I had been both a watchmaker, an computer engineer over my lifetime, so the iphone surgery wasn't too worrying. Armed with a grounding strap, bright desklamp and the ifixit-supplied toolkit, the whole procedure worked first time - took about 40 minutes. I really took my time to do it right, first time - tensioning screws properly, keeping contacts and seals clean, and fingerprints off all surfaces and contacts. Phone powered up on first attempt, and all peripherals (camera, sensors, buttons, speaker etc.) worked perfectly.
Things missing from the ifixit guide:
1: Antistatic precautions - MANDATORY for low-voltage electronics, and not mentioned. I'm sure this is responsible for a lot of the failed attempts at repair. I used a grounded static strap clipped to my wrist.
2: It's darn hard to avoid bumping the power button, thus turning the phone on whilst disassembling or reassembling it.
3: Reputedly, The "do not remove" sticker mentioned in step 11 must be re-affixed on reassembly (or replaced with a suitable equivalent) . If not, the phone may randomly reset due to internal shorts. I was able to remove it intact, and re-use the sticker.
4: Ensure the "tang" on the top of the glass face is in the case slot BEFORE closing it back in the case.
5: To restore full battery life, I had to factory-reset the phone, and re-download the installed apps.
6: at no stage was force required - if force is required at any point, you're doing it wrong, and something will break.