When they release seats that they're holding for revenue depends entirely on the flight and route. I've seen AA release seats slowly, one or two at a time, between 72 hours and 3 hours, and I've also seen them hold an enormous block of seats for sale until 4 hours before, then release all but 3 or 4 for upgrades.

The bottom line: in my experience, you still stand a good chance of clearing the upgrade before the 3 hour window. After that, you'll have to wait to DFW to get on the upgrade standby list. (The "DM list" in the impenetrable jargon of AA employees and JIMBOLIGUY.)