As long as you download them in the month they're free - then you have access to them for the entire period of your subscription to PS Plus. You will lose access to them if you cancel PS Plus, but if you sign up again at a later stage you can get all the old games back.
I swapped my hard drive for a 500GB one so I just download all of the free games each month to play at a later stage. For only R600 per year I think its great value.
