@riccardo-brizzi I think in order for Google Maps to give you a time, it requires to be in the navigation mode, which from what I recall is not possible with pre-defined routes.
The Google MyMaps tool is only to show a map with your route or track, but cannot enable you to navigate the route using GM.
A quick google search re-confirmed my thoughts and findings from years ago: https://support.google.com/maps/thread/202415953/how-do-i-make-a-kml-route-navigable-in-google-maps?hl=en