In the beginning,mine would quit at any throttle position. My opinion is that it wasn't drawing fuel, and going lean. I never felt as if an onboard glow would help a fuel starvation problem. However, to answer you question, anything below 1/3 throttle is where I would have the light on. Over that, and it should not need any help staying hot.

Depending on you application, 1/3 throttle may be just about right for normal cruising. So, another way to look at it is fly the model, then see where the throttle stick is when you are at a power setting that is somewhat lower than normal cuise flight. Make the glow ignite at that point. That way, it's not on most of the flight, wasting your onboard battery.