The only C++ M1 time and battery benchmark I remember is from Matthew Panzarino[1]. He found that it took the M1 Air 25 minutes to do a fresh compile of WebKit, and that the Air had 91% battery remaining after finishing the compilation.

So, roughly assuming that you could compile WebKit 10 times before the laptop runs out of juice, that'd give you 4 hours 10 minutes of battery life.

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[1] https://techcrunch.com/2020/11/17/yeah-apples-m1-macbook-pro...