How long will a frozen lamb roast stay "good" ?
It will be safe indefinitely at freezer temperatures.

For maximum quality, the frozen shelf life (per Still Tasty) is about 6-9 months.

You would want the lamb well wrapped, so that freezer burn cannot develop.



It appears that at the time this question was put in, your lamb is about 4 months frozen, so it should still have good quality if well wrapped.