How much time does Apple needed when replacing the iPad's battery?

You can make an appointment (*) in advance (just to be sure the battery is available) and have it replaced at an Apple Store or AASP (Apple Authorized Service Provider) among this list (select your country):--> https://locate.apple.com/choose-country-region , and get an estimate online here:--> Choose Your Country or Region - Apple Support.

Apple technicians will replace the battey in 1-1.5 hours, barring inconveniences.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

For practical purposes, in lieu of replacing the battery or repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair.

If the iPad is not exchanged, an iPad battery will not be replaced in-store. Instead, the device will be sent to Apple's service partner. As the time for the round-trip may be protracted, Apple generally prefer to exchange the device - as already detailed.

As for battery replacement and out-of-warranty repair costs, Apple offers a fixed-price price structure. You can verify eligibility and cost for your region/country here:

iPad Repair & Service - Apple Support

If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.