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The time from sending a SL command to it becoming “active” is 80-100 ms . Also remember that before giving SL command modem needs three character long break.
SL commands have to come from DTE(data terminal equipment)which means that it is not possible to send SL commands from radio modem to another radio modem.  So therefore you should send a command that activates a local program to change the frequency, this prevents the frequency being changed inadvertently through spurious data caused by interference
 

To view the calculations to work out transmission delays click HERE

In addition, Satel technicians add.....

“80 - 100 ms for the frequency change has been a value with quite large tolerances. Due to re-engineering and further tests, Satel will eventually shrink the Tx/Rx channel switch time down to 40 ms.

Case#1 - Controlling the channel change by SL command (SL&F or SL&+ or SL&-)

- After sending the SL command to the modem and receiving the "Ok" from the modem, the terminal device may send the actual data message to the serial port. The modem will add an extra 40 ms delay to the transmission of this message (= the next Tx message after the channel change). It means that the point-to-point Tx delay for that message is the normal delay + 40 ms.

- CTS will be ON all the time. No additional timing is needed at the serial interface. The modem is ready to take in the Tx data just after printing out the "Ok" message.

Case#2 - Dual Channel mode set ON (= "Dual" or "Reverse dual") in Dual Band or single band unit.

- An additional Tx delay of 40 ms will be added to every transmitted message, and therefore seen in the point-to-point delay too.”

Also.......

- After the previous transmitted message, equal or more than 3 ms break time has to precede the SL command.

- 3AS modem will respond with "OK" or "ERROR" after receiving the SL command

- 15 ms after the last byte of SL command, the next data string may be sent to the serial port for transmission.

- In case of a frequency change SL command, however, no radio transmission will take place during the time period of 40 ms from the last byte of SL command. So, data sent during the period of 15 - 40 ms after the freq.

change SL command, will experience an extra transmission delay. But if it is sent after the 40 ms period, point-to-point transmission delay will be normal (see Appendix B of the user guide)

- CTS will go down at the end of SL command, as it does after any data message sent in via TD line.

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