Answer
A newly applied speed limit should be saved immediately if it is edited in MyGeotab, or typically updated once per month if it is edited in OpenStreetMap (OSM).
Additional Notes
- Also see MyGeotab: MyGeotab: How to update posted road speed limits
- OSM used here are on Geotab's servers. When a speed limit is changed in a database, a call is made from the database to Geotab's Map servers to propagate this change to the entire map federation, which means this road speed change will be reflected in all Geotab database. When you check the speed limit for this road on a any other database after the change, it will state "Speed limit was changed on XXXX/XX/XX (date)" with no records in the audit log about this change.