Also, it is different for the amount of beans, size of bean and the density of the beans. This will depend on which process you use. When the beans start crackling, check their color every two minutes until the bean has achieved the color slightly less than the color of beans desired. This is because of the exothermic process. When the beans cool, they are still roasting.
Some people recommend that a bean of proper color be set aside for comparison.
However, the problem with that is, those who know enough to have done this before, probably know how long this process takes any way. Another problem is that the inside beans never roast the same as the outside beans unless turned. This is where the art form comes in. It for sure is guess by golly for the first time roaster. When the beans reach the desired color, or degree of roast, take them out.