AVT Reliability® recommend having your MSM-01 calibrated no later than every 3 years. This is included in your license cost so there is no extra fee to pay.
The MSF-01 do not need calibrating.
A sensor calibration due date can be seen in the administration section of the web portal. Click on 'Contracts', navigate to the correct hierarchy level for your site where the MSM is used then click the MSM Sensors tab, you can then see the 'calibration due' column.