When you get below half a chamber of tobacco and ash, how do you know when the smoke is over? Certainly, the level of the ember is the first clue. When you get below a third of a full chamber it's time to pay attention. Having to relight more often is also a signal that you may be reaching the end of the bowl. Tamping gives you a sense if there is some springy unburnt tobacco left, or if you are just tamping mostly ash. The primary signal is flavor. If you are still getting the full harmony of tastes in the blend, you are still in the zone. When flavors begin to fade and/or have a slightly bitter or harsh edge, it is time to scoop out the ash and dottle (unburned tobacco) if any. This is no time to listen to your inner thrifty parent and keep relighting a bowl of ashes. just let it go out or stay out and scoop it clean. Flavor and enjoyment are the objectives, so when that ends, think about the next bowl.