How and how long should lamb be kept frozen?

Frozen  lamb will keep indefinitely at a temperature of 0°F or lower. For best quality, use frozen roasts and chops within 6-9 months and ground lamb within 3-4 months.
 
For freezing, lamb should be tightly wrapped. Prepackaged, raw, unfrozen lamb sold in stores may be wrapped using a process called modified atmosphere packaging (MAP). The air in the package is replaced with gases such as nitrogen that don’t interact, like oxygen does, with food components.  Products wrapped this way may be puffed up on top by this gas. Lamb should not be frozen in this type of packaging because water can migrate from the food to  the head space (the bubble)  created by the gas; the result will be dehydration and freezer burn. Also,  the plastic wrap over the head space can tear if something heavy is placed on it. Then product will dry out and develop freezer burn. 
 
For long-term freezing, overwrap the store plastic with freezer storage wraps or bags. This will prevent freezer burn, which appears as grayish-brown leathery spots and is caused by air reaching the surface of food. If defrosted lamb does show signs of freezer burn, it does not mean the meat is unsafe to eat. Just cut these portions away either before or after cooking. Heavily freezer-burned lamb is likely to be of poor quality, so it should be discarded.
 
Source(s):
USDA Fact Sheet "Lamb From Farm to Table"