I've had skins that were over a year old that were only skinned, and not fleshed in the freezer, that turned out just fine. The most important thing to remember is to keep the freezer bag as air-tight as possible. You don't want the skin to dry out, and you don't want it to get burnt. Remember that the skin will dry out alot sooner if you don't have the body attached to give it some solid bulk to aid in the prevention of drying out and freezer burn. I will skin out my birds, and freeze them, then I'll flesh up a few, wash them, and leave them wet and freeze them wet in a vacuum sealer. They will last for years this way, but you will need to wrap the feet in a wet paper towel to keep them moist as well. A little side note here for you hobby taxidermist/hunters (like myself), a skinned bird (in your freezer) still counts as your possession limit, so make sure you mount them up in your off season.