If you check the C:\Program Files\Common Files\Symantec Shared\Licenses folder you'll come across an slf file(or files if you have multiple Symantec Products on this machine).  If open them with notepad you should find one mentioning either Brightmail Antispam or Message Filter and you should probably see a line in the file mentioning an 'end date' or something similar.  That should let you know when the license will expire.

Kevin

You can also search for registered licenses at:

   https://licensing.symantec.com/acctmgmt/index.jsp

There you may find licenses associated to your company.

Carlos Oliveira