My new brushless setup for e-maxx

[suicideneil]

Thats not a terribly common motor choice for 6s lipo- you will need to gear rather high to obtain ~40mph, which will hurt your efficiency and run time. You would ideally want a ~1600kv motor for 6s lipo & 40mph topspeed- the 1515 2.5D (1650kv) would have been the optimum choice really for an MT, lots of power and great runtime.

Runtime in itself is very difficult to guess, it depends on many many things like specific battery choice, terrain, driving style, ambient temps, vehcile weight, wheel/tyre choice and so-on. Best guess- with decent lipos and under racing conditions, something around 20-25minutes sounds about right.