Thank you both for the reply. I think that sounds right - If I can't knead it then it's time to throw it out.
They say it can last forever but after wrapping it up tightly in a lightly greased bag I would toss it within three months. The grease will keep the outside edges from drying out and causing all those pesky little bumps in the fondant.
No problem. Our kids just covered a board with leftover MMF. I could be wrong, but I believe it was 9 months old (unrefrigerated)... worked like a charm--just like or better than fresh!