COMAND Maps update and TomTom Live Traffic data are both free for the first 3 years from date of first registration.
New COMAND navigation maps are realised roughly once a year. So for the first 3 years you should get 2 free updated (in year 2 and year 3). However, members' experience shows that unless you insist that the dealer updates the COMAND maps during servicing, more often than not they do not bother. So you really need to be on top of this, i.e. check what is the latest release of maps available for your car, insist that the dealer carry-out the update for free, and then double-check that the latest version has indeed been applied before accepting the vehicle back. Sad but true.
As for the TomTom Live Traffic data, once activated it will just work in the background, until the 3 years expire at which point you will need to purchase a 1-year, 2-year, or 3-year subscription renewal. The activation and renewal are done via the Mercedes Me online portal (requires a Mercedes me account and registering the vehicle to it).
EDIT: Mercedes do say that for the first 3 years maps update can be downloaded for free via the Internet connection on the COMAND Online unit, or by downloading the maps update to a USB memory stick at home then uploading it to the car, though members here reported this process to be often fraught with problems.