Yep. Been there. Done that. Or, tried to.
As is the case with so many things, it depends. Economy of scale will GREATLY reduce the individual time necessary to complete a pen. If it's a "one off" fountain pen, it might be an investment of several hours over 2, 3 or 4 days. There's always the time to procure materials (computer time), driving to and from the Post Office, etc. Plus the time to get the incidentals necessary like adhesives, abraisives, finish, etc. Really hard to account for all that. Then there's the easy part of turning, finish and assembly.
HOWEVER - I once had a 100+ pen order (another story) and I thought that it would be a good opportunity to do a test. I tracked my hours and compared that to the end product which was a variety of single and two-tube pens. All wood. Mind you this is just the basics. Pick a blank, drill, fix the tube, mill the ends, mount, turn, sand, finish and assemble. All told it came to about an hour and a half.
That was a while ago. I'm faster now.