Hello,
There is a warn period on the license for 30 or 60 days I believe. Then there is a grace period for another 30 or so days where the program should still function. Past that Brightmail is supposed to fail open, so it should continue to pass mail but without new spam definitions you will most likely see spam increase very quickly.
You can view the license file in a text editor for end dates, although it is a little less than intuitive. The end date is when it expires entirely, so minus the grace period and such. I would suggest that you contact Symantec or your reseller to begin negotiating your contract if you are going to stay with Symantec.
I hope this helps.
Tom