With the new rules a scale flight could be as short has 10 level laps and landing or as long as the rules allow. On the B-29 I can typically get two contest flights out of 4000 Mah sized batteries. It all depends on what throttle setting you are at during cruise, if you cruise at 1/2 power it will go far. The only real way is to fly it and measure the flight times.
If you look at the battery dimensional sizes there is a small difference between a 3000 Mah and a 4000 Mah. I use the hobby king website to get a gauge on the dimensional size of the batteries once you know the voltage, either 3S or 4S. The more the Mah the bigger they are in overall size.
If you can build and then test you can go up or down on the Mah size. All of my future CL scale models are going to be electric, I still have some glow powered sport models.
Fred