20 - 30 minutos
My phone began crashing left and right, often with 65% left on the battery meter, whenever doing anything power intensive like taking a photo. I decided that it was likely due to an old battery on it's last leg.
The internals of the phone were smaller and trickier to work with than I thought they would be. As long as you're careful and take your time it should be fine.
I've seen several horror stories online about batteries which are not compatible with IOS4+ and result in endless reboot loops. I emailed ifixit on a Sunday evening to inquire about compatibility and received a prompt reply within a couple hours assuring me their batteries were compatible. Good customer service, would order from ifixit again.