You should for sure reach the First Crack. Less then that will still have some grain-like tastes... As you keep going the sugars in the coffee will begin to caramelize like Cracker Jacks or Creme Brule. As you go further you trade the sweetness and varietal nature of the coffee...
By now you’ve probably already tried roasting some coffee. Hopefully you’ve got a roaster that really lets you see the beans roasting. They start a pale green, then get to a khaki color. As the remaining moisture leaves the beans, it smells kind of like hay. They gradually turn tan, sable, and then brown.