How to create a 30min timer for each session under serverless

Use EventBridge Scheduler. When you have a new user session create a schedule for 30 minutes from now. When it fires, invoke a Lambda that clears the session.

I second the solution provided by Uri of using EventBridge Scheduler. I would like to add in addition you can maintain user session in DynamoDB. When the user starts a new session, create a record in DynamoDB table with appropriate startTime, expirationTime , userId, sessionId values. When the user ends the session or when the session expires, you can delete the corresponding record from the table.