Originally Posted by Globaliser

You must request the voucher before check-in closes for the flight. Normally, this is one hour but I have lost track of whether there are any exceptions to that at present.

A couple of my requests have been made relatively close to that deadline (I think about an hour or so before it, so about two hours before the flight). On one occasion, I got a rejection email so I had to do it again. On the other occasion, the voucher confirmation email didn't arrive until some hours after the flight had departed. If those two problems had combined when I did one late, I might have permanently lost the value of the ticket. So you can leave it until quite late if you like, but you may need strong nerves.