For new Chromebooks activated after August 1, 2014, you have 180 days to redeem the offers once the first account is created. If you miss this window you are out of luck. The free storage lasts for two years from the date you redeem it.
Late last year, Google had a 1 TB free offer, but that had to be redeemed before 1/1/15. You still can get 100 GB of free space though.