The nature of my business requires frequent last minute changes/cancellations, and I've seen credits post anwhere from immediately to 7+ days. Based on my experience:

If changed within 24 hours of booking:
If Ticket Paid for with CC: Usually takes 3-5 days to credit back to my card
If Ticket Paid for with My Wallet: Usually deposited about 3 days later
(Note that if I know that I need to use the funds soon, I always opt to change the reservation rather than cancel so that I can use the funds immediately)

If changed after 24 hours of booking:
Any form of payment: Depositing into My Wallet has always been immediate. Emailing the ticket in the form of a voucher is usually instant, but has take up to 30 mins. No experience with refunding to CC in this case since I always opt for the credit.

I'd be interested to hear of other exceptions!