Hello,
There is no confirmed date set on when would the SAV 10 definitions would stop getting created. If the Extension is purchased for the term of 12 months term then it is assured you to receive the definitions for 12 months.
The Virus definitions are also available on the Symantec site for Manual download.
However, as far as I understand these definitions are not available after the EOSL date from Symantec Liveupdate server unless you make a request for the definitions extension.
Have you applied for Extension to the virus definitions?
How to request a virus definition extension for Symantec AntiVirus 10.x Corporate Edition beyond its End-of-Support-Life date
http://www.symantec.com/docs/HOWTO73168
Extensions will be available for 1, 3, 6, 9, or 12 month terms starting at the EOSL date.
Hope that helps!!