Tiger was Spring of 2005, Leopard was Fall of 2007. That one was delayed several months because Apple had to divert resources to ship the first iPhone. Snow Leopard was Fall 2009, then Apple basically moved onto the yearly release cycle in Summer 2011 with Lion and moved it to Fall of every year with Mavericks in 2013.
I believe Mavericks was also when Apple stopped charging for OS upgrades.
I believe Mavericks was also when Apple stopped charging for OS upgrades.