In the past electric models were simply flown or driven until the battery waned. This was fine for NiCd and NiMH packs but doing this to a lipo can greatly reduce its life. As such it is common practice to follow what is referred to as the 80% rule. This is not a hard and fast rule exactly but more of a recommendation where only 80% of the rated capacity of a lipo is used up in flight.
The 80% rule
This rule protects both the lipo and the heli. Lipo voltage under load is fairly flat during hard discharges until you reach about 10% remaining capacity and then it drops very sharply. If this voltage drops too far it can damage the lipo. Likewise since voltage is directly related to motor speed, this sharp voltage drop will also cause an equally sharp drop in rpm and that can effect the heli's flight.
The 80% rule is applied by keeping track of the mAh used during a flight and keeping it below 80% of the rated capacity of the pack. That 80% is calculated by
80% of pack mAh = pack mAh * .8
For example here is 80% for the following packs
3s 850mAh 15C lipo
850mAh * .8 = 680mAh
3s 2200mAh 45C lipo
2200mAh * .8 = 1760mAh
6s 2200mAh 20C lipo
2200mAh * .8 = 1760mAh
6s 5000mAh 25C lipo
5000mAh * .8 = 4000mAh
Important concept: Note how the number of cells and other ratings of the pack make no difference for the 80% rule. Only the capacity of the packs is used.
How to calculate % mAh used after a flight
So how do you figure out what % of a lipo has been used after a flight? You charge the lipo with a smart charger capable of displaying the total mAh put in during the charge. That number represents the mAh taken out during the flight. To find the percentage
(mAh used / lipo capacity) * 100 = % used
For example take a 2200mAh lipo. Say you use this pack and recharge it to find you put in 1550mAh. The % used is
(1550mAh / 2200mAh) * 100 = 70.5%
Note on fuel gauges
Some chargers come with what is often referred to as a "Fuel gauge". When you go to charge a pack the charger will give you a % of remaining capacity and then as you charge, it will rise to 100%. These are rough estimates only and can be very inaccurate, in fact they usually are when checking depleted packs. This is because they rely on battery voltage to estimate capacity and that is generally inherently inaccurate, greatly changing from pack to pack. As such, these fuel gauges should basically be ignored and should never be used for the 80% rule.
Determining flight time
Now that you know what the 80% rule is and how to calculate the % used during flight, lets put it to use. First you will need a flight timer. Some radios have them built-in but if your radio does not, a digital egg timer or other cooking timer can be used. Start by setting your timer for 3min and then fly for those 3min. Next you recharge the flight pack to learn how much mAh you used. From there you can make a rough estimate of how long a pack will last.
For example lets say you have a 450 sized heli that uses a 2200mAh 3s pack. You set your timer and hover the heli 3min. When you recharge the pack you find that the charger put 761mAh back into the pack. Since you are aiming to use no more than 80% of the 2200mAh pack, or 1760mAh, you now know you can more than double your flight time. So you go out and hover for 6min. That time when you recharge you find that your heli used 1568mAh. That tells you that for hovering 6min is very doable.
Another approach is to fly for a known length of time and then divide the mAh replaced by the charger by the minutes flown. That will give you an average mAh/min number that you can then use to determine max flight time. Using the numbers from the above example, a 6min flight used up 1568mAh. So each minute consumed 261mAh. Dividing 1760mAH by 261mAh will tell us how many minutes the heli will fly while consuming 261mAh per minute. That time is 6.7min.
Flight time is a moving target
Now just because you figured out the flight time once does not mean it will always be the same. Any change you make to the heli, be it mechanical, electrical or the way it is flown, will have an impact on its power consumption. Some changes will lengthen your flight time but most will decrease it. Some will be slow to happen and others will be very quick. As such, you should always be monitoring the power consumption of your heli. Most of the time simply checking the mAh needed to recharge the packs each time will let you know if you need to make an adjustment to flight time, but once in a while a large change is made and you will have to repeat the process above again in order to figure out what the new flight time should be.
For example a common 450 sized heli will hover for about 10min on a 2200mAh pack using the stock motor, esc, and an 11T pinion to achieve a head speed of about 2650rpm. A simple pinion change to a 13T will raise the head speed up closer to 3000rpm and drop the flight time to about 5min, all while leaving everything else the same. So when making changes be sure to make a short test flight, not only to test the new setup, but also to figure out the new flight time.
Following the 80% rule is just good practice and now that we have fancy radios with timers built into them, it is easier then ever (no need for an egg timer clipped to your radio :). With a little practice and habit, you will be able to fluidly adjust your flight time as needed and that will hopefully keep you and your heli happier.