Free members only get 30 days worth of accessible storage. The games aren't erased beyond that month, you just can't see them unless you go premium. Let me see if I can find something official for you...
http://support.chess.com/Knowledgebase/Article/View/179/3/why-cant-i-see-all-of-my-games-history
Hi! Does anyone know why games I played before 2013 are not shown on my log?
Thanks!