I wonder how English-speaking people can describe time periods in this particular case. The amount of work to be done is big and will surely take three days but how many days after it is not known. Supposing, there are two situations: when asked privately (1) and when as an employee (2). Can you say:
1 a. I'll be doing this (for) more than three days.
1 b. I'll get it done after three days.
1 c. The job will be finished in about three days.
2 a. We can do this for you in three days.
2 b. We're working on it from three days.
2 c. The task will be complete in more than three days.
It is also interesting if the expression is natural or at least understood: “It takes three-plus days”. Do you use it and how is it good stylistically then?