I built this quad a few years back to use as my normal drone to race around and bash, but a month ago I made a li-ion battery and was shocked that I could easily get, even with playing around, 23 minutes of flight with a 4s1p 4200mah 21700 li-ion pack and almost 30 minutes with a 4s2p 9100mah pack. With more research I discovered that almost no one has been able to accomplish this with a non-optimized racing drone.

Total weight and flight time:
*quad w/ out battery: 343g
RDQ 4s 1300mah: 506g ~ 6min max
diy 4s1p 4200mah: 657g 21-24min
diy 4s1p 9100mah: 959g ~ 28min

The "small" 4200mah battery is made from straight P42A 4200mah molicel li-ion cells. it should handle a constant 45 amps and I have pulled a peak burst of 70 amps. The "big" 9100mah battery pack is made from 4x molicel in parallel with 4x panasonic 4900mah cells which adds some unique and quarky features, but for an fpv application it would work best with straight molicel cells which would give a 8400mah rating, but really close to the same flight time due to the higher c-rating

For motors I am using the dji snail 2400kv motors with a 2305 stator. These motors have a higher efficiency than any other fpv motor I have found to date. Just to put into perspective howefficient these are, the most efficient T-motor with a 5" prop makes less than 3.5 grams/watt at 150 grams of thrust. This dji snail motor on the other hand is rated at 6.2 grams/watt at 150g thrust with a 5048 prop and my testing has verified these claims to be pretty accurate. I am no motor expert, but I have been building electric paramotor prototypes for an established paramotor manufacturer, so I do know the basics and here are some unique things about these motors

Stator: 2305- more pancake than almost any other fpv motor, helps produce torque more efficiently at low power. It's also capable of swinging lower pitch 7 inch on 3s.
Winding- this motor has by far the cleanest and most dense winding of any fpv motor I have ever seen.(see pictures) This helps a lot with efficiency and power. Some companies like T-motor have come close to the quality of windings on their top of the line motors like the 40pro, but they still have a ways to go.

I have also been super impressed with the efficiency of the dji snail 5048 tri-blade props. I have tested nearly a dozen 5" props both 2 and 3 bladed, high and low pitch, and from multiple manufacturers and none have pulled as hard, been as quiet, or needed as few watts to hover or cruise around as these. The downside is that they weigh more than most other props (nearly 5 grams each) and they have become harder to find.

Conclusion: I would probably say that this favorite build as I can slap an 4s 1300mah on it and keep up with almost any non-purpose built race drone. But when I want to go long range, I can mount my 4200mah or 9100mah battery, crank my vtx up to 1000mw, and I can fly longer than almost any 7 inch long range quad while still getting about the same performance.

*Update: I was doing more research to find alternate motors to the dji snail. I came across the MAD srs400 2400kv. I have bought industrial size motors from in the past so I can speak for their quality. Anyway at 340g thrust per motorwith a 5050 3 blade they are rated at 5.9g/watt efficiency which is right where the snail should be at that thrust level. They also have very comparable high density windings and weight to the dji snail.

*Update #2: With my 4s 9100mah battery I just got 31 minutes 37 seconds hovering which is why I updated the number in the title.
Also in a previous flight today I tried to kill my 4200mah battery as fast as I could... It still lasted 20 minutes. I'm not a super good pilot, so I am sure other people could do it much faster, but it's still hard to believe.
finally, this whole li-ion battery think has gotten me hooked on endurance quads, so i just got motors/props today for my next build. I don't want to spoil too much, but the four motors, 5 inch props, and all related hardware is only 36.5 grams! My goals may be lofty, but i want to try and break the world record for the sub 250g quad endurance which i believe is currently 72.5 minutes. I also want to break the world record for the lightest flying weight of a 5" quad. I'm not positive on the current record, but I estimate that i can get under 120 grams with a 2s 300mah battery including a full fpv system. :)

*update #3 12/9/2020: I recently got some VTC6 cells to build another 4s battery pack. The finished product should weigh less than 400g, but give 6200mah for ~27 minutes of hovering. I am also upgrading my 4s 4900mah to a 9800mah which will hopefully give 35 minutes of hovering or 26+ minutes of playing around.