You have 2500mah which is 2.5Ah x 60 is 150Aminutes. If you put a watt meter between your battery and esc you will know the amps used at different levels of throttle. If you want to be really conservative you could start with max amps used at full throttle. If this number is say 25 amps. Then dividing 150Aminutes by the number of amps drawn / 25A you get 6 minutes of flight at full throttle. You would actually get a bit more flying time as a static test of amps will be higher compared to a plane that is already moving and the motor is not working so hard to turn the prop. You don't want to run your lipo totally dead so stay conservative.

Alternately you could charge the battery fresh. Fly for 5 minutes. Recharge the battery on a charger that shows the number of mah going into the battery (Triton charger) or use any charger and put a watt meter between the charger and the battery. If in 5 minutes of flight your battery requires say 1250mah to be recharged in full then you used half of the 2500mah available and could have flown for roughly twice as long (10 minutes).

I like the magister in stock form so it must be a hoot with brushless. I have flown the stock motor with a esc that allows lipo on a 3 cell lipo and it's great.

Carl