The question is often 'how long does it take to implement'. Engineers know if it is more or less clear what the functional requirements.

In practice, workdays are constantly interrupted by meetings. An 8 hr workday is effectively 2-4hrs product building. Than there are unforseen deployment problems, some requirements change, solve merge conflicts, perform code reviews, frameworks need vulnerability updates, etc.

An experienced PM knows this and should not use implementation time as project duration.