20 - 30 minutos
Seemed like my battery on my iPhone 3GS was lasting a New York minute. So decided to replace. Apple store would cost $79 plus deprive me of the phone's use for 5 days. Enter iFixit for $19.95 plus loose change.
Yikes! Even my engineer/modelmaker husband found it a challenge. I gave up after I couldn't budge the phone apart with the suction cup. An hour later and all was successfully concluded, but.....
BTW, I easily increased the RAM on my Macbook Pro previously using iFixit.
Marry an engineer/modelmaker. Barring that - be prepared to be challenged. Not the fault of iFixit. It's just a tough switch out.