Originally posted by xx:

Hm. I will look more closely, I could've sworn I had some crops dissapear after coming back to my fort after being away for several days

some of your plantings could have been too close to each other...they will turn brown almost immediately. The brown ones will eventually die. Also, as mentioned above, any plantings that do not have direct access to the sky will die (an oak tree branch for example will block the sky).

But any crops that are planted correctly, and spaced correctly and not damaged by mobs in anyway should last indefinitely.