I have no idea, but based on history:
Tiger was originally released on 4/29/2005, and received its last security update on 09/10/2009 (~4 years and 4 months; Source: http://support.apple.com/kb/HT4218)
Leopard was originally released on 10/26/2007, and received its last security update on 5/14/2012 (~4 years and 6 months; Source: http://support.apple.com/kb/ht1222)
Lion was originally released in July 2011, which implies you should be good until around 2016 or so, but again you never know. It is yet to be seen what the change to a yearly release schedule for OS X will do to Apple's obsolescence schedule in a few years. That is, if OS X even still exists in a few years. 😉