Are you using the old (free) version of UserSync? It’s been abandoned years ago. There is a new rebuild version of Usesync in the Premium add-on, that works a little different. The new version doesn’t subscribe or unsubscribe emails, it only updates existing contacts. Opt-in/subscribing needs a clear opt-in moment eg via the user registration integration you can find under MC4WP > Integrations
The old free version of usersync is no longer supported.
There is no build in way to do the sync all on a schedule, and you shouldn’t normally ever have to do that. It should automatically sync any changed users every hour.
If you are using the new UserSync that is build into the Premium add-on, please contact our Premium support if you are having any issues with it. Thanks!