The voltage by itself is not a good indicator of a battery being fully charged. Rather, how much current does it hold. It will quickly show the correct voltage under no load. The correct test is to measure it under a typical load of 4 servos etc.

Either way, if your battery is say 500mAh, and you use your wall charger, typically 50 mAh, then you would need to charge for 10 hours to fully charge it. If you have a fast field charger, then this time is much less.

-tyhcoc