How many miles can you get out of a super shoe before it loses its effectiveness?

Go out a bit quicker, somewhat close to even pace for your targeted time, assuming you feel OK doing so.

In my experience , as prescribed by the brand , in the 500 to 600 km range

Do a few (2-4) workouts in them before you race. Doing so may even increase running economy, as getting the lacing right will improve the fit and decrease foot movement in the shoe.

In my experience, the shoes tend to lose a bit of pop at around the 200k (125 mile) mark. However, this is just a subjective feeling and may differ from person to person, and shoe to shoe. I tend to continue wearing the shoes for training (sometimes up to 400k), but won't race in them after the 200k point.

I read a study that compared super-shoe efficiency between the brands. The study also tested a pair of of the original alphaflys that had done 200k with a fresh pair, and it found no decrease in running economy.

Wear them pre race and wear often. Don't worry they last a long long time, it's a myth that they die after a few races. They're great for workouts and long runs. Just enjoy them.

You definitely don't want to use anything that has "very different feel" in a race. Get used to it first; that's what training is for.

Also use the training to determine the proper pace for the race.

I generally no longer race in mine (Alphafly, Vaporfly) at around 200-250km. I will use them for training for another ~300km after that.