Service Battery issue
Wait depends on two factors: a) whether the Apple store or the service provider has the battery in stock; wait for delivery from Apple is probably a day or two; and b) the existence (or not) of sufficient techs at the store or shop to do the work that customers have ordered. If work is backed up, it'll take longer.

A battery change should take .5 - 1 hr, and if the customer is lucky (i.e., battery in stock and a tech available), might be able to be accomplished while the customer waits.