An employee always receives a notification when their mission is launched.
If they haven't completed it, and depending on your settings:
a reminder 14 days before the deadline;
a last reminder 3 days before the deadline;
social nudges, triggered by the progress of those around them: their squad or a rival squad reaches 80%, the company reaches 75%, or a first member of their squad finishes. Their squad leader can also nudge them.
Each of these reminders can be disabled in Settings → Cyber Mission. See 3️⃣ How to Configure Cyber Missions?.