In our software we have the ability to import and export to Revit. We have implemented that using the API. Currently we support 2009, 2010, 2012, 2013 and 2014. But maintaining support for all versions takes quite a bit of time, so I wonder how long time does autodesk support old versions? Infinite? 5-years? Or maybe there is a list somewhere?
To answer your question, most firms that are not sole-practitioners are on subscription, which means that they can use up to 3 previous releases on their license. That is, those who have upgraded to 2014 can no longer use 2010 or prior releases.
There are some people who are not on subscription, although they are becoming fewer and fewer, and are unlikely to be in your target market (an assumption, since I have no idea what program you represent), so I would say that you can discontinue support for 2010 and prior releases, and next year discontinue support for 2011 and prior releases, etc.