I also had gone through the cancellation & voucher recently. Took 2 days to arrive (Qatar has an FAQ somewhere that says 5 days, the chat support tells you 7-21 days ...). It had +10% value, and after booking a new flight I have ~$50 left on it and the booking page did clearly display this and the FAQ also says that a remaining value can be used on another booking.
But you can't do this twice, flights booked with a voucher cannot again be converted to a voucher but only refunded (less the fee, while there is none at all when converting to a coupon).

But I do also asked myself the question that this is quite generous by Qatar, hopefully not misused by people.