Are Exchange server policies permanent?
No, they are not. For as long as you have the account registered in your email app, the policies are applied, however, if you remove the email account then the policies will no-longer be enforced. 

The app is given a "device administrator" status (you can see any apps which have this in "Security" in your settings), and you can revoke that at any time. Revoking it, will of course, cause your email to stop working.