I had an unopened one that I bought last Feb., opened it and ate over the week-end, it was good, like I just bought it. Made them into Marzipan candy. I feel fine, no food poisoning or anything like that even now.
As long as it is well wrapped and sealed it keeps indefinately. Its only air that can ruin it. The best way to tell is to knead it. If it quickly becomes smooth and soft it's good to use but it you feel little hard bits in it the air has got to it so don't use it.