RFID sources and battery lifetime
I once designed a small transmitter that woke up every minute or so to measure temperature and transmit the value along with an ID. These were used in a room which had a stack of freezers and if one of the received transmissions indicated the temperature was too high an alarm sounded. Collisions of data was expected but each transmission only laster 40 milli seconds every minute or so if you had 100 of these all randomly transmitting, the total transmission time of all one-hundred was less than 10% of one minute. There was also a random factor built in based on ID.

Battery life was paramount on this but 1 year was got (from memory) from a PP3 style 9V battery. The engineering to get more battery life is keep the electronics powered off as long as possible, ensure the quiescent current (when off) is as small as possible and keep the transmission as short as possible. I used 433MHz off-ther-shelf transmitter modules and one off-the-shelf receiver acting as the central collection point for data.

Incorporating a receiver is not a good idea i.e. transmit on request would need a receiver that takes up valuable real-estate and consumes power almost constantly.