This might be a silly question but i was wondering if it would be a good idea to delete all expired \"sessions\" from my database, every 15 minutes?
Or just leave it
If you're using the standard SQL Server Session State, old sessions are automatically deleted for you. Until the old entries are deleted, sessions do not expire, since the provider does not actively monitor the Expires column in the session table.
The install / configuration utility creates a SQL Agent job that calls the DeleteExpiredSessions stored procedure every 60 seconds. This means that SQL Agent needs to be running for session expiration to work and for the table to get cleaned up.