I always look to Exchange as the support deadline and per this:
Microsoft Support Lifecycle:
It’s all the way to 2020.
Also some things to think about…
You now get 2 virtualization rights with Windows Server 2012. So if you want to build your own SBS equivalent with an on premise Exchange, you get the right to install Windows Server twice in a HyperV not just once. (Source see http://www.aidanfinn.com/?p=13042 )
There will be a “thing” that will allow you to grow past 25, keep the wizards and the RWA – see http://blogs.technet.com/b/sbs/archive/2012/07/05/windows-small-business-server-essentials-becomes-windows-server-2012-essentials.aspx for a hint of it. Also read about how Windows Server 2012 Essentials will support an on premise Exchange. So while there is disruption, it’s not as bad as the Home Server folks and enthusiats that now have to figure out what their next home NAS unit will be.