The amount is constant for each person, but one person might have a 2 hour performance window while a 100x engineer might have a 9 hour performance peak.
You can increase or sidestep it a little with performance-enhancing drugs or a healthier lifestyle, but with their downsides, changes in your environment and other aging-related issues you'll see over a long enough period that it doesn't change much.
You can increase or sidestep it a little with performance-enhancing drugs or a healthier lifestyle, but with their downsides, changes in your environment and other aging-related issues you'll see over a long enough period that it doesn't change much.