It should clear at the sweep, which unfortunately UA only does in the middle of the night (I believe). It's possible for R availability to open and then close between daily sweeps, so make sure to monitor it. In my case, I kept watching it daily, and then when I saw it open up I called to have an agent remove an expiring GPU from a reservation which I presumed would fail, and clear me into J on this flight. (I had her put an RPU on the other flight, and it failed by a country mile.)