By using the following method, transmitting time can be extended to more than 5 minutes. (5 min max transmitting time in continuous mode).
You will need a receiver and a transmitter both with off delays.

Set the timer on the receiver and the transmitter so that the delay off in the receiver is more than twice to that of the transmitter. In the example below we have used 2.5min off delay on the receiver and a 1min off delay in the transmitter. The transmitter will transmit every 1 min which will keep resetting the receiver timer.

If for some reason (power failure or interference) the receiver does not receive the first signal, the second signal (on 2 min mark) will reset the receiver timer. If there is no signal received after 2.5min (transmitter is off) the receiver will switch off.