Tips for those building a quadcopter from scratch #1.

Weight is your enemy.  Consider the weight of EVERYTHING.

You can typically lift about 0.25 oz/Watt.  That means that one pound of aircraft will 'cost' you 64Watts. If you have a 3000 maH 3Cell battery, that is 3A X 12V = 36 Watt/hours. Most quads will weight between 2.5 and 3.5 lbs (battery included). 

So, if your quad weighs 2.6lbs (such as a Cheerson CX-20), a 3000 mAH battery will give you 36/(64*2.6) = .216 hours = 13 minutes of airtime. If you add a gimbal and a GoPro Hero3, you add approximately .5 lbs.  This will decrease your airtime to 10.8 minutes.

Of course, the aggressiveness of your flying will cause these numbers to go up or down, but you get the idea.

Adding weight will also decrease your horizontal speed.  As the craft tilts, the lift decreases.  More weight means that you can't tilt as far while maintaining altitude. Not tilting as much means your horizontal speed will decrease.

Buy or build the lightest, strongest frame you can and also buy a kitchen scale and an electronic fish scale so you know how much your individual components weigh. Even the wiring adds substantial weight.

A good rule of thumb is that you want a motor thrust that is twice the "All Up Weight" (AUW) of the craft.  Another rule of thumb is that the batteries should weigh no more than about half the AUW (or equal to the weight without batteries).  So, if you have a 500g battery, your airframe should (ideally) weigh no more than 500g and your total thrust should be at least 2000g.  

But, it is hard to find an airframe that weighs only 500g, since your motors probably weigh 100g (or so) each. Your airframe will probably weigh closer to 1Kg.  So, for maximum airtime, you would choose batteries that weigh 1Kg (maybe two 4500mAH, 3 cell batteries in parallel?) and the thrust per motor would need to be 1Kg (4Kg total).

For the above craft, the weight would be 70.4 oz, which would take 70.4 * 4 = 281 Watts, and the batteries would be 9AH * 12V = 108 Watt/hours.  That would give an airtime of 23 minutes.  Not bad!