Usually battery replacements take a day or two(most of the time you get your MacBook the same day) depending on the availability of the replacement right away. You can book an appointment online and then head straight to the genius bar or someone who's standing at the door who'll lead you to the genius bar. You can also apply through mail ins but that takes about 4-5 days to ship it back. You can check more on https://www.apple.com/support/macbookair/en/battery_body.html
You can book your appointment online. They are really good about seeing you on time so there shouldn't be a long wait.
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