Fondant- how long does it stay fresh?

If it is a commercial fondant then you should be able to email the manufacturer and ask this question. 

There should be either a date of manufacture or a "best by" date and this will depend on your country's food regulations.  The date of manufacture may be coded. 

When you buy fondant, you should also mark dates onto the container--your date of purchase and your date opened.  That way you will be able to know the rest of the history. Mark the dates onto any bags if you split the container up.

If you open a fresh bucket of fondant and immediately wrap cake-sized portions into double freezer bags, you can put those  back into the bucket and have the longest shelf life possible.   You should use a newly sanitized knife and board to cut up the portions.  The commercial restaurant grade sanitizing solutions are the best. Shelf life then depends on temperature of the room where you store it. 

The expiration date on the commercial fondants I've bought is at least a year from my purchase date. I've had fondant pass its "best-by" date and I've used it to cover dummy cakes for practice, so if it goes over, it's not a total waste. To be perfectly honest, I've tried pieces of this left-over fondant that's as much as two years old (from a recenly-opened container, it hasn't been out for two years) and I find that it tastes just the same, has the same consistency and I've suffered no ill effects from it. I would never use it on a real cake if it had passed its expiration date, but I've often wondered whether those dates aren't too conservative - or mandated by law, as the previous poster suggested.