Depends on the condition of the battery to start with - both health and state of charge. If the battery is near zero and in good health, the surface charge gets absorbed quickly until the battery gets close to 70% or so - then you would need less time to understand what's going on. If the battery is near full, the surface charge takes a while...Ours started the car in the mountains after being left overnight, at 13F, when the resting voltage was 11.2V but it cranked really slowly and probably wouldn't have lasted long if the car hadn't started right up. FYI, I'm testing the voltage with a USB adaptor in the cig port, not directly on the battery, and with ACC on, so there's other load from the HU starting up, interior lights on, etc.
rlouie, how long should a battery be left after charging (i.e. driving the car) to get a good sense of the true state of charge?
Example. If you have a 12.0V reading on your battery, and you charge it for an hour at 14.4V/5A, immediately after charging stops, you will see that voltage drop to under 14V and then quickly through 13V (probably less than 30 minutes) where it starts slowing at about 12.5V. Given a few more hours, you'll see it drop down to approx 12.1V
On the flip side, if you have a 12.7V battery and you charge it, immediately after charging stops, you will still probably see the voltage drop to under 14V but it will be a much slower decline to 13V (may take 2 hours) where it slowly declines down to 12.88V in another few hours.