Your answer is in the Help Centre - please check there first when looking for answers:
https://www.airbnb.com/help/article/544/guest-refund-policy
This means the guest, if they cancelled, may not get a refund at all but travel credits for a different booking. This is up to Airbnb to manage - not you as the host.
If the host cancels they have to request a cash refund from Airbnb - again they will manage this and when it is given back to them is on their timeline.
https://www.airbnb.com/help/article/170/what-happens-to-my-payment-if-my-host-cancels-my-reservation.
If you have strict policy, they may not get a refund at all depending on when they actually cancelled. https://www.airbnb.com/help/article/149/what-is-the-airbnb-cancellation-policy