If it can be cancelled online without manual intervention (which seems to be the case most of the time right now), then the Avios will be back in your account within a few minutes. You may need to log out and back in to see your Avios balance updated, however.

If you see a somewhat convoluted-looking form while trying to cancel it online, it means your booking cannot be cancelled without some manual intervention. It means you may need to wait for up to a couple to get your Avios back.

If you telephone to cancel a redemption they may be able to process the Avios to be put back in your account even if a manual intervention is required. However, I am not sure it it would incur an offline service fee.