If I would travel to palm sea from a region next to it for instance, that's 6-7 turns for me and 6-7 turns back towards my own lands. So imagine if we go a region deeper into enemy territory, where I have no say over where market places will be. Even if I post up buy orders before I leave, my lords will be unable to sell their food.
I have to admit that if you plan a bit ahead you can store enough bushels, but it's not fun/easy/enjoyable to have to worry about such things. Which should also a part of the game I think.
Okay, so 14 turns--7 days. A region deeper--let's give it another 4 days. That's...11 days. Offers stay on the market for up to 14.