Hi,
I have checked this question out a number of times in the past and can find no concrete evidence of any inherent time limit on these cards, one reference I found quoted some 25000 reads/writes but i couldn't confirm that anywhere else,
I totally agree with keeping backups but in answer to your original question i don't think they are time limited, just like any other storage including hard drives I would, as I am sure you already do, keep backups as often as you can, as that blokes law always seems to come up and bite you when you are least expecting it
