Generally speaking, your first set of coals will die before the bowl is cashed. It all depends on the size of the bowl, the tobacco and the coals you're using. You really need to learn this on your own through experience. Most times as the first set of coals is about to die you can add 1 or 2 new coals and extend the session, but the tobacco may begin to lose flavor and you will end up wasting coals. Again, there are many factors involved and you will need to experiment to find out what works for you!
Hope this helps some.