How long will it take to replace battery if I have an appointment in store

If they have it in stock, it doesn't take long. If they don't, essentially what they do it give you a virtual place in line among customers at that store. They'll contact you when they have one for you and you have many 3 days to make a drop-in visit for the installation. There are reports that some people are being told it's taking longer at some stores than others. I suspect that the allocations of batteries are based on location, but it would be difficult to know where to go where you might get it earlier.

Depends how busy the store is, roughly 2 hours.

iPhone Battery & Power Repair - https://support.apple.com/iphone/repair/battery-power - battery replacement and cost (also talks about repair time)

Hi

You need to make an appointment at Apple store.

To make sure battery is in stock.

Cheers

Brian 🇬-1F1E7;