Originally Posted by Aeroplayin
Charge your battery
Set your timer for 2 minutes
Go fly normally for the full 2 minutes
Recharge your battery and read the mA that you returned to the battery before you disconnect the battery from the charger - most chargers will display this.
Take the mAh rating of your battery and multiply it by 0.8, so if you have a 3000mAh battery, then 0.8 x 3000 = 2400. This is 80% of capacity because you don't want to completely drain your battery.
Divide the mA you put back and divide 2400 by this number
Take that number and multiply by the 2 minutes to get your projected flight time
So if I flew for 2 minutes and put 800mA back into the 3000mAh battery, then 0.8 times 3000 is 2400, and 2400mAh divided by 800mA is 3.
3 time 2 minutes is a projected flight time of 6 minutes.
Heyy... that's my line!
Also, when you come down, check your voltage. If you have 3.7 volts per cell, that's borderline. Try and adjust your flightime (using 75% capacity instead of 80% mentioned above works best, I've found) so you land with about 3.75-3.8 volts per cell. This will extend battery life (life as in number of cycles).