How long does it take to replace battery in iPhone

Physically it takes less than half an hour. But how long it will take for a particular customer depends on way too many factors including whether or not the correct replacement battery is in stock, how many technicians are available, and how many other jobs are in the queue to be performed. I personally waited about 90 minutes. My dad came late in the day and they couldn't get it ready until the next day. I've heard of the battery not being in stock and then it took time until a shipment with that battery comes in.

Depending on how busy the location is, up to a couple of hours.