Extreme Networks supports firmware for five years after the hardware product end of sale. For example, v8.3 was released 9/2014 and has a End of Support date for 9/2020.
Using v7.4.2 as an example:
v7.4.2 was released: Thursday, May 26, 2011
End of support would be: 5/2016
A reference to this information can be accessed through this direct link:http://learn.extremenetworks.com/rs/extreme/images/Extreme_Software_Status.pdf