After some months of low cost low power room sensor usage here is the feedback.
The sensor send every hour to OpenHAB with the OpenMQTTGateway temperature, voltage and humidity data, it is powered by 3 AA alkaline batteries.
But the most interesting thing about this sensor is how the battery voltage decrease so as to have an idea on the batteries lifetime.
Here is below voltage (mV) graph on the last day, month and 4 months sent by the sensor (the charts are updated every day):
We can see that on a 4 months duration the battery decreased around 0,1V. If we consider an end of life at 3V for our 3 AA batteries this give us a lifetime of
4,5V - 3V = 1,5V
15 * 4 months = 60months = 5 years of expected duration (without taking into account the AA auto discharge)
5 years of expected duration with a less than 8€ sensor !