I believe it's between 10 days to 2 weeks. If you were the buyer who has contacted Clickbank for a refund, Clickbank will then send an email to the seller and give them a week to respond. After that 7 days period, a decision will be made and a refund then be processed.
For some reason, the Clickbank Return and Cancellation Policy doesn't mention this. However, there's a helpful article on Clickscoop which goes into some detail on the subject of Clickbank refunds: Understanding ClickBank Refunds.