It's 24 hours for USA flights in theory but 36 hours on connections I think. Now I am not sure about the overnight component, but for the London-Lisbon section you should get an email a few minutes after the 36 hour point. I would wait for that since there is a URL in that which neatly takes you to the right place. My guess would be the USA service will be greyed out at that point, which it wouldn't be if same day, but you can go in via the same email when you are within the 36 hour point and see if you can get it to move. You need to be on an ESTA for this to work, the mechanism isn't set up for visas. You also need the usual API information, notably your address for the first night in the USA.