That will be somewhat of a SWAG. I'd start by running the engine around and measuring the current draw.
Let's say that in typical operation, you were drawing round 1 amp, I'm assuming you might be using the smoke unit. 
For each hours running, that would be 1000 ma/hr of battery capacity. If you pick this 2400 ma/hr battery pack, you'd get around two hours of running. The nice thing about Li Po batteries is they keep their voltage up for most of the discharge cycle, so you can use most of the battery energy.
Obviously, the 1A is just an example, I've never measured the LC or LC+ stuff to see what it actually draws.
