Quoted:
35-40 inch pounds.

One thing that rarely gets mentioned, as most AR15 manufacturers don't even seem to know or care about it is that the Mil-Spec has used an aviation-grade high-temp tolerant gasket between the key and carrier, made from Permatex Form-a-Seal compound.  Some people like to use Red Loc-tite, or even black.

You degrease the carrier and key thoroughly, then acetone-bathe, while only handling with gloved hands.  Test-fit the carrier key, carrier, and gas tube beforehand.  Use US-made Grade 8 fasteners, and apply sealant to the fastener threads as well.  Once torqued, use a compression staking tool, not peening.  Peening can break your seal of the key to the carrier.  I use the Brownell's tool, and many like the MOACKS.

A quality carrier key is a different animal from many of the keys I have seen over the years.  There is a certain hardness to the steel, it is hard-chromed inside, and the dimensions need to be within a tight tolerance.  If the Inner Diameter of the key is too large, you can have short-stroking.  If it is too tight, you will have the flanged end of your gas tube worn down, and the gun is likely to short-stroke.

I saw this recently at a 3-day DMR course that I ran.  The culprit was a 20" Bushmaster.  The gas tube flange was worn down on one side.