Do you have a charger that measures how many mAh you put in the battery? In that case, there's a neat trick I learned here on Wattflyer and modified a bit that works great:
Make sure you have a full battery. Pick a really short flight time, like 3.5-4 minutes. Bring her down and charge up the battery. Check how many mAh the charger needed to put in to fill the battery to capacity. Let's say the charger put in 1500mAh. That means you still have 1500mAh left, in other words you have 50% left. Next time you fly, increase the flight time by about 25% and try again until you've reached a point where the charger tells you it puts back about 80% (2400mAh) in the pack. That leaves you with a 20% margin, which is optimal for making your batteries last and leaves you enough spare capacity to handle a go-around or two. Keep an eye on the number of mAh you put in the battery for the next several flight in case you need to fine tune your flight time.
Edit: According to the manual, the stock motor (not sure if that's what you have) has the model number KM0374810. If we google that, we find the following data:
Code NoKM0374810Specification / SizeD37.2 x 48.5 mmPackage1 pc/pkgDescription-Max Speed: 13860rpm
-Kv (rpm/V): 770
-Operating Power: 450W
-Operating Voltage: 8.5-18V
-Operating Current: 30A
-Peak Current: 38A (max. 15 sec.)
-Internal Resistance: 34 m ohms
-Shaft Diameter: 5mm
-Mounting Screw: M3 (Front) and M2.5 (Back)
-Distance of Mounting Holes: 25mm (Front), 15.5mm (Back)
See the full page here:
So the maximum continuous current is 30A, which would mean you'd drain 3000mAh in 3/30=0.1hr=6 minutes. No idea what you draw at full throttle (only way to find out is to measure with a wattmeter), but if that is indeed the motor you're running, you should easily get a 5 minute flight out of it as long as the current stays within the recommended limits.