There's a really easy way to calculate this on your own. Each NiCd cell should read around 1.3v (probably a little less) off the charger. Once the cells drop below 1.2v it's time to recharge.

CG is correct. There's no such thing as over-amping. mAh is simply a measurement of capacity with respect to time. 1000 mAh means that the battery can draw 1A (1000mA) for an hour, 500mA for two hours, etc... Voltage is like the force that pushes the current (amps) through the conductor. Too much voltage can cause damage, but that's it. The battery chemistry simply creates free electrons for power.

When testing, I usually put current meters in the airplane circuit to calculate how much draw there is. You don't have to, but I am finicky.

The reason the voltage drops is because it is mathematically linked to current. Voltage = current x resistance (V=IR) If you lower current while maintaining the load (resistance), then voltage drops.