I will add, besides the studs replace the yokes, washers and nuts too, I had quite a few of the stainless conical washers that were cracked in half.
Also I was talking stock engine, so once you start winding in some boost, manifold and turbine temps go up and the heat expansion will be greater, so they may only last 40 000k
Another thing, when studs break in the alloy head, the part of the stud which comes away, nearly always takes the last one or two threads with it, don't panic there are plenty left, you'll junk the head before you need to worry about helicoils.
You will also be tempted to over tighten the nuts which is a bad idea, as it will stop the exhaust manifold from sliding over the washers and yokes and put more stress on the studs. I use a open end / ring spanner (the ring end has offset) to tighten them, the open end will just fit the 1/2 inch drive of a Tension wrench, so set the torque you require and feel with you hands how much force is required through the spanner to reach the setting, then try and replicate that exact force when tightening up the manifold, you will be surprised how little force is required.