During development, coffees begin to develop and open up by beginning to caramelize and releasing sugars and different chemical compounds. At first crack, we know that the bean has begun development. We know this because the beans begin to crack like popcorn. Ideally, we try to avoid a rolling crack when the beans crack at different times and instead aim to obtain a roasting curve that allows all of the beans to crack at once. Usually the first crack happens somewhere between 6-7 minutes.
Depending on the level of roast we are aiming for, development times will vary. For light roasts, we will typically end the development right after first crack. For medium roasts, our development time will be longer by a few minutes. For espresso and dark roasts, we will typically hold the beans inside the drum right before or immediately after the second crack.