Not so bad I think, if its from parts and you're assuming an expert gunsmith(which can be assumed here), you start from the barrel(a tube of the suitable metal), the firing mechanism(standard gears and spring assembly, common to most guns), chop it down to size, etc.
I'd say from parts or proximate parts(that is, stuff you can cut, file or otherwise slightly reshape to qualify), it can take hours. The long slow process is more when you have to produce things like barrel grade metal in the 'field'.