@d00m We’ve been able to run a simplified version of the loraWan stack from an ATtiny85 in combination with the RFM95. (deep) sleep current is less than 1 uA.
So if you use a 1200 mAhr lithium thionyl chloride battery (size 1/2 AA), you can keep the device in sleep mode for about 50 years! 
(actually it’s over 100 years but the battery loses 1% energy per year) 