Now, data rates of IEEE 802.15.4 are of 3 types-250 kbps, 40 kbps, and 20 kbps and the transmission time varies accordingly.
Here's the formula if you want to calculate it:
Time(s) = Data(bits) / Rate(bps)
For example, if the rate is 20 kbps, the data(message) is 200 Bytes, the time taken is given by,
For 250kbps the time taken is 6.4ms.
PS: Actual time may vary as processing time is not taken into account in this formula.