Back when I bottled, I would have one bottle in a plastic bottle, so I could tell when it was hardened up. I would leave it a little short of a full bottle, squeeze it a bit to push out most of the O2, and wait for the shape to return by CO2 production. Then when sufficiently hard to the squeeze, I would chill it and try it. Most of the glass bottles would be ready at that point, too, but I would usually give them a little more time, just to be more sure.