A manager should have at least a vague notion of how long a given set of tasks should take an employee.
If I assign 20 hours of work and it takes 40 hours, I should not be surprised the employee does something else with the 20 remaining hours.
If I assign 40 hours of work and it takes 40 hours, and is of the expected quality, I really don't care if the employee take a part-time job elsewhere (assuming that doesn't conflict with expected online hours, etc).
You are either managing factory workers, or living in a dream world.
My boss has no idea what I'm working on, day by day. My assignments take as long as they take; he makes WAGs at the project start, and I get informed if I am burning more hours than he expected.
But at the end of the project, a "40-hour" project may take me 10, or 100 hours. Or, rarely, 40.
But overall, I think I have a feel for how hard my employees are working and whether they're completing things in a time I consider reasonable. Maybe I'm being played. I dunno.
If I assign 20 hours of work and it takes 40 hours, I should not be surprised the employee does something else with the 20 remaining hours.
If I assign 40 hours of work and it takes 40 hours, and is of the expected quality, I really don't care if the employee take a part-time job elsewhere (assuming that doesn't conflict with expected online hours, etc).