I flew LH492 last Aug 4. I used upgrade award certificates for my companion, and was in the waiting list since the ticket was issued end of Jun. The waitlist was clear Jul 31. I called the Senator line in Germany and she was assigned seat 83H next to mine (83K). Guess what? When we checked-in at Senator Lounge in FRA (we connected from AMS), the agent told us that 83H was already occupied by a VIP. I argued that I had called the Senator line 31 Jul, and was "confirmed" for seat 83H. But the agent said loudly that my companion was not "booked" for the upgrade. I politely asked her to check the reservation, and I saw the PNR clearly shown in F. Anyway, we ended up moved to other seats to be together.

If the seat map still shows several empty seats, most probably your upgrades will be clear several days before departure.

Last April and May my waitlist was cleared about 4 days before departure.