How long it will take to replace my iPhone battery?

Depends on too many variables. They could be whether or not the battery is in stock (might need to be ordered or delivered from another store), the number of available techs, the number of service orders before you, etc.. One possibility is that they might offer a full replacement device in lieu of a battery replacement, so be sure to have a backup.

I got mine in under 2 hours. My dad got his the next day when he went to an appointment late in the day. But it could be days or over a week if they don't have the correct battery available.

My own battery replacements (4x) have been pre-scheduled and in an AppleStore.

Time “in” to Time “out” has always been under 90m.

Your experiences might vary.

iPhone Battery Repair & Replacement - Apple Support