Paul, getting to first crack in 6 minutes is a positive. No need to become concerned over that.
What youve read about first crack in 3 minutes generally represents the frustrations of roasters who wish they could slow things down.
However, you are obviously worried that the extended time--while being good for the coffee roast --may be outside the rated operational time of the popper. Out in the open, where temperatures are probably half those inside the house, overheating should not be a problem.
But if you are nonetheless still concerned, there are ways to speed things up. Keeping the lid on, and using 100 grams of beans should do it.
By the way, how many watts is the machine?