While management may know what time should be spent on a project, there are several variables that affect the time spent on the project by staff. Variables like, rest, personal issues, time of the day, distractions, politics in the office, internet speed etc all have an impact on the time spent on completing the project.
Any manager in the service industry will tell you that it is almost impossible for staff to meet all their time budgets on their projects.
So instead of measuring the input that your team is putting into each project rather focus on the output that you expect from each team member to meet their salary costs each month.