Using a loaded voltmeter (like the Hobbico meters), it's generally accepted that when the voltage reaches 4.8V, it's time to recharge. By that time you're somewhere in the "meat" of the charge, but you could be just shy of the drop-off point too. Unfortunately, you can't tell where that is and if you go long enough to find out, it'll be too late. So, charge when you hit 4.8V on a loaded meter.