There is no exact measurement in this case but you could get an idea by testing certain types of implementations. For example if you are finishing similar tasks on average 25% faster during a longer testing period with and without AI. Just the act of timing yourself doing tasks with or without AI may already give a crude indication of the difference.
You could also run a trail implementing coding tasks like leet code however you will introduce some kind of bias due to having done it previously. And additionally the tasks may not align with your daily activities.
A trail with multiple developers working on the same task pool with or without AI could lead to more substantial results but you won't be able to do that by yourself.