Like mentioned above: that depends on the spreading factor. For 18 bytes of application payload, on SF9BW125, a total packet size of 31 bytes needs 246.78 milliseconds time on air.
What did you measure? What module is it?
Assuming US915, your airtime should never exceed the maximum dwell time of 400 ms; for 18 bytes application payload SF10 is not even allowed. So I guess you know that whatever you’ve been measuring is not just the transmission time. However, if there is no downlink in RX1, then I’d also expect the module to try RX2, so the full cycle without a downlink would take the time on air, plus 2 seconds, plus a bit for the preamble detection to know that there is no downlink in RX2. For that, 1.52 seconds seems too short.