I had a friend into model railway steam locomotives - he always said that when he retired, he would build one. I went over some maths with him.
We guessed the engine (Evening Star) would be made of probably 5000 individual parts.
If you worked on 1 piece at a time and turned a piece of base material into a fully finished part every day, it would take almost 14 years - non-stop, every day, no holidays.
if any part could not be finished in a day, the project would drift on longer.
He bought a kit instead - fettling & painting only.