I just called QR Priority Services in Doha yesterday to request for an upgrade using Qcredits LHR-DOH-SIN. Travel date ~ 1 month away. Availability J6 on EF at the time of request.
They put me on waitlist and the upgrade was confirmed within an hour or two. Both the waitlist and the upgrade confirmation changes were reflected on the MyFlights app when I refreshed the booking.
I recall being on waitlist for up to 2 weeks with other requests in the past. Guess it all depends on how far away the flights are, and whether they think they can still sell revenue tickets?