You need to use the Semtech ‘LoRa Calculator’ tool. Not sure where it can be downloaded these days, Semtech seem to move it around.
This website does a reasonable on-line job, it seems to agree with the Semtech tool when I tried it.
https://www.loratools.nl/#/airtime
Most of the LoRaWAN\TTN defaults are listed, so you need to supply the spreading factor used and the packet payload length in bytes. This will give you the on-air time in mS.
Fair use limit is 30 seconds of air time per day.