I've had this situation many times, as the route I fly most often (BOS-LHR) never has any C inventory (in advance) on the direct flights.

My strategy is generally to avoid the busiest flights (e.g. Sunday overnight, Friday evening), and to look for I and D inventory.

Usually, the upgrade clears 3 to 5 days in advance. Sometimes a bit longer, but that seems to be about the expected time. I've had one clear the day before, and so far, I've been lucky enough to have never missed a requested upgrade.