More than 100 hours. (Putting in the request for an upgrade using 500 mile certs when I bought the tix, not clearing and being priority waitlisted at the 100-hour mark, not clearing at airport arrival, having the flight delayed, and not clearing at the gate.) Using SWU's, HK-49's or miles, I've almost always selected flights on which I could confirm the upgrade at purchase--if the flight's long enough to merit using one of those upgrades, I don't want to risk being stuck in the back. The few times that hasn't been possible, the upgrade cleared in advance or by the time I got to the airport.