All depends on the time of the year and how much heat we get weather it ferments or not.  This is where individaul wrapped square bales shine over anything else.  They are packed so tight, that we don't usually have any problems with it keeping at any moisture as long as there is plenty of plastic on it and no holes.  We put 10 layers on everything.  I've wrapped 35% moisture stuff and drier this late in the fall and opened it up in March and its the most beautiful, green, sweet smelling hay you will ever see!