There is no set date, other than the one set by MS, but that didn't stop avast! from supporting older OSes when they were no longer supported by MS (win9.x being a point in question). There are people still using win2k and that is working with avast 8.
It isn't so much the OS but supported avast version, avast 4.8 still have VPS definition updates (but no program updates) and it has been on life support for some considerable time. However, that switch could be thrown ending 4.8 support and the OSes that can run it win9.x.
So avast has a long history (excuse the pun) of supporting old OS versions, but it isn't total support. As new functions/features are introduced into new versions of avast, some elements may not be supported in certain OS versions as they simply don't have the OS elements required to run the new avast function/feature.
So obviously those new avast features won't work in older OS versions, but the other older avast features should still work. But the most important thing is virus definition support, if there is a change in the way virus definitions are delivered. If that changes then that could be an issue, but looking at the duration of support for 4.8 definition updates have continued it could still be available for some time.
But as in life there are no guarantees and things change, which is why there is no firm date at this time.