Some of the members of your project team are not just assigned to your project but to other projects too. How do you estimate how long their work will take?
You don’t. You can’t. Don’t even try.
Multitasking is a guarantee that your project will be late. You can’t tell by how much the project will be late, because you can’t know how much time every person can spend on your project, and you can’t tell whether the people who need to be on the project at the same time will actually be there.
You cannot estimate the schedule if you have multiproject, multitasked people. (Multitasking can waste anywhere from 20--90% of your time; see Ship It! A Practical Guide to Successful Software Projects [RG05] and Quality Software ...