Well if you look at this calc program, it give rough ideas of flight times. I have gotten longer flights then it lists. One said I would get about 17 minutes of mixed flying and i went over 30 minutes.
I have a Monster Power 46 with a 5s 5000 mah swinging a 13X6 APC E prop. I get 10 plus minutes of mixed flying easy.
The calc programs says 10 minutes on a 13X6, 12 minutes on a 13X5.
Are you setting this up with a battery per motor in each nacelle? or one central battery?
http://www.s4a.ch/eflight/motorcalc_e.htm
Buzz.