You are assuming a very simplistic design to these when they are out of the case and idle. The most likely scenario is that even during that first two hour window, the sensors know that the device is not being worn and not moving. Thus it is largely 'off'. There is no active noise control or other sensing going on, so the only things being powered are the accelerometer and low powered bluetooth to maintain the connection. Modern BT is so low powered that I have devices that last years from a coin cell.
Again, the most likely scenario is that after that 2 hours, the BT connection is shut down and again, the only thing powered is the accelerometer which is also incredibly low powered. I will bookmark this post because my bet is that standby time in this state will be measured in weeks if not a month.
You know what? My kids bluetooth headphones (which were all relatively expensive for kids headphones) have physical on/off switches and guess what. They never turn them off. The batteries are dead the next morning, and they are scrambling to charge them before school starts.
Apple's decision to leave out the power switch and use the sensors that are more than capable of detecting when the device is in use and when it is not makes sense. It will prolong battery life in many situations, and greatly improve the user experience all of the time.
I will enjoy coming back to this thread when the above scenario is borne out by the full reviews.