No, you can't define an alert. What would be the use case?
You can define a conversion goal for user sessions lasting longer than 30 minutes. But you will not get an alert, you can just than filter for that user sessions.
Thanks Alexander. The use case is that the customer experienced some security breach and just want to know if they can use Dynatrace to track down all users having a session that is more than 30 minutes. And they want to be alerted to monitor those user sessions if there is /are something anomalies going on.
The only idea I have for alerting in such case is exporting live user sessions to external tool like elasticsearch and there validate conversion goal (as Alexander said). From that point you should be able to send alerts.