According to the http://www.rfxcom.com/descr_RFXSensor433.htm :
When the high rate setting is used the RFXSensor unit will measure the temperature for each attached sensor every minute and report any changes of .5 degrees C or higher. If a change of .5 degrees or higher has not been detected for 40 minutes the current temperature will be transmitted as a keep-alive or heartbeat signal. This setting allows for frequent temperature updates but will consume more power and result in a shorter battery life.
When the low rate setting is used the RFXSensor unit will measure the temperature for each attached sensor every 5 minutes and report any changes of 1 degree C or higher. If a change of 1 degree or more has not been detected for 80 minutes the current temperature will be transmitted as a keep-alive or heartbeat signal. This setting provides less frequent temperature updates but will provide the longest battery life.