Dunno how underestimated it was, it depends on the plane and the people doing it. One year is overestimated to say the least
My fastest was 9 days for an external for Il-2, it wasnt a hard one I give you that. The cockpit needed 1-1.5 months but it was more like a reconstruction than simple modeling. Of course for BoB it will be more, but in a year one could build the entire Luftwaffe with photoreal details