It can, in theory, be done at any time.
The risk with Email Comms is that a user cancels a batch while the system is in the process of dispatching it automatically. In this case, while awards should still be cancelled, there's a risk some employees still get notified.
There is not a straightforward way to predict when this may happen: once a batch is created, the system does not indicate when exactly it will begin dispatching. As a general rule, dispatch begins with the first cron job scheduled to start after the batch has been created. Dispatch cron jobs are hourly, so it may take between 1 minute and an hour for the dispatch to begin.