A drafting supervisor of my acquaintance estimated that his people could produce two drawings per day, regardless of size or complexity. That was before CAD, and included simple A-size sheets, too. I'd figure a day per drawing sheet.
In the case of an add-on, someone is going to spend at least a person-day, probably two, just measuring and sketching what's there already.
Mike Halloran
Pembroke Pines, FL, USA