Originally posted by photoptimist 64GB is my max but I prefer using 16 GB cards and rotating them each day of a trip to reduce the chance that card corruption or camera theft means I lose all my pictures.
(I've taken all my older, smaller 1, 2, & 4 GB cards and put one each in each of my camera bags, fanny packs, & backpacks. That way if I forget to put a card in the camera (or it's full), have a card problem, or whatever, then there's always a spare card in every bag.)
I copy all files to a laptop every evening, and then to an external drive I have with me. As a computer professional, I know from experience that many backups are the best protection from corruption, failure, and theft.