Currently, "Continuous" mode syncing syncs 100% of the time. However, in many cases, there are idle periods in which continuous syncing isn't needed, followed by periods in which smooth syncing is necessary.
The VRC Object Sync component does this exact behaviour, going to zero bandwidth usage when not moving, and having the ability to do this for UdonBehaviour variables would be very useful. This is particularly the case when the UdonBehaviour is attached to an Object Synced object; right now, this results in greatly increasing bandwidth usage as the Object Sync data is no longer able to go to sleep.