What is the timeline for returning my MacBook Air?

Assuming you mean return after having purchased it and not for trade-in, like all purchases you have 14 days to return the purchase for a full refund.

click here ➜ Returns & Refunds - Shopping Help - Apple

Please do not post your serial number here.

This is a user-to-user forum, not Apple.

What’s your question?

Apple publishes the general timeline for the trade-in, and provides return-related materials.

The return can be made by shipping the device, or directly at an Apple Store.

Once the return is received and evaluated, the payment will be transferred.