Basically, as long as 9.4 is supported. But you may be asked by SAS support to upgrade to the current ML if you encounter problems that have been fixed in the meantime.
Since no 9.5 or 10.0 is on the immediate horizon (AFAIK), you can safely bet that with an upgrade to M6 (current) you'll buy yourself support for a decade (out of my experience), or as long as you have SAS licensed.
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