I have the Phantom vision plus and it is advertised as 20-25 min flight time. In the real world I average 15-16 mins landing with about 25% battery remaining. I have a phantom 1 and fc40 and with the 9450 props I average 12-13 mins and 10-11 minutes with the standard 8" props before the red light starts flashing (I believe it is set at 25% Batt remaining) which is pretty accurate to the advertised 10-15 mins.
What flight times are you averaging before landing and how much battery is remaining? DJI advertises 18 mins so Im guessing 13-14 mins at 25% battery remaining?

Your guess is correct. It also depends, if you take off to 20ft and just stay there hovering you might get close to the 18 mins as advertised. But under normal conditions I bring her in after 11-12 mins with around 30% battery left.

12-14 mins at low tempratures (from -2 to +5 Degreees) is a realistic time at the moment.
But i think i have always returned before 25%.

All my flights so far have been 10-12 minutes shutting down at 20-25% battery. Temperature has been quite high though

You may get a better response and answer under the Phantom Forum where those people are that have plenty of experience with the Phantoms. My experience with my Phantom 2 Vision has been an average 25 to 30 minutes of flight time. What I do is fly and when the Alarm comes on I keep flying but within reach until the Battery hits a level of 7% and land it and let it run until it stops. That is bringing the Battery full circle. et it sit for a bit to cool off and then recharge.

I have been seeing 12 minutes in cold weather with the smaller battery which is not the best.  I have a Hex with a GoPro and the A2 system that never gets less the 18 minutes with a 5000MA battery.  I hope the larger battery and warmer weather on the Inspire adds to the flight time this summer.

Get the TB 48 5700mah it should do about 16 to 18 minutes flying smoothly .

I did some calculating based on my flight times and the 18 minutes seems to be a good number for the TB47 if you were to fly it all the way down to 0%.  I divided my flight times by the proportion of the battery used to determine what flight time would be if I had flown down to 0%.

Example:  flight time = 13:25 and landed at 20% so 13.42 minutes / 0.8 used = 16.78 minutes if I had flown to 0%.  This was on my longest flight which was 22,977 feet long.  I've had quite a few flights calculated at just over 18 minutes but probably averaged around 17 minutes with this calculation.

I used to fly down to 0% with my Phantom quite often - don't think I'll attempt that with the Inspire.  

jhannon66.gmail Posted at 2015-2-12 10:58
I did some calculating based on my flight times and the 18 minutes seems to be a good number for the ...

Your calculations are well intentioned but likely incorrect. Flight time down to 20% of 13 min+ has been my experience too.
However, battery discharge curves are NON-LINEAR so the extrapolation down to 0% is likely incorrect.
As the constituent cells hit the lower end of their working voltage, they deliver less current. This is why the % indicator falls more quickly as you get to 25% reserve.
LiPo s should be deeply discharged occasionally but to go down to 0% or even 10% on a regular basis will shorten the life of your battery.