I’m guessing that with the new $29 battery replacement program every Apple store will have a battery replacement assembly line in the back room. Having replaced my own 6 plus battery a few weeks ago, I’m confident that with the right tools and some experience a tech can bang out a new battery every 15 minutes or so.... just a guess though.
by law, rechargeable batteries can't be shipped with more than like a 20% charge. so by the time a store gets it and uses it, the dang thing is likely at like 2%. unless they are total morons there's no way they will give you the phone back in that condition so you can count on a good 20-30 minutes of it being charged and then diagnostics being run again before they will say it's ready.
and I highly doubt any big assembly line in going on at anything but the biggest stores. they aren't likely to have tons of parts in and the repair areas probably aren't that big and are being used by folks doing computer repairs also. at least they might have like 3-4 folks at the same time in a medium sized store. those tiny little ones that haven't been remodeled yet might have only 1 person doing all iPhone repairs
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I would have thought giving you a replacement phone would be problematic. To make it completely functional it would need to be a Clone of your existing device or at the very least restored from an encrypted copy of your existing phone. Even then it would create problems for Find My Iphone and 2 step verification which requires a trusted device. Hardly worth it for a day.
replacing a phone, when it's justified, isn't a problem at all. They make you turn off find my iPhone before they do the swap and 2step can use a text or a phone call. as for clones, that's your issue not theirs. they don't backup devices to anything in store. you have to have backed it up on your own computer or done an iCloud backup that you verified completed and nothing was turned off. For about the last 3 months or so they actually make folks, at least in the US, sign a waiver before they will even run diagnostics that they have zero liability for your data.