balsa fire -- you do not have near enough battery capacity ( my opinion) for your setup -- further - a 6v setup is better for servo response.
There is simply NO way to predict what the flying time span is for your particular battery /servo setup -
too many variables.
There are a number of guys in your area flying the Spektrum systems- check around and get some informed info on battery drain and servo/ flying combos
PS the stock 4 servos and the stock battery are fine for a simple sport model but as soon as you increase numbers and types of servos and IF you are setting up what is referred to as 3D throws - you need far more capacity-