How do I determine appropriate deadlines for my employees?

Do you remember Detective Columbo? He would scratch his head and say "I am confused, can you explain this to me?" Well, it seems to me that you need to use the Columbo technique.

Put the employee in the position of getting you up to snuff on this project so that you can document procedures, tasks and the time involved with the project.

Let the employee know that it is your goal to incorporate this information into an action plan (a quasi manual) for the future. Remember, you are asking for assistance from this employee, rather than confronting him.