If indeed it is meant as the time it took to establish or set the assignment then for me “in” is most correct.

How do I make the ability to speak English more fluently and effectively, including teaching Spanish and French?

Instead, a time assignment like this might be intended to mean the “duration” allowed, allocated or given to complete the task in the future.

Can we know whether a task gets done or not from the context? The intended meaning is the time given for the task instead of setting the assignment, then I would recommend use of the word “duration”, e.g., when my job is done! “Estimated Duration: 10 minutes”