Boris,
I have restored a few. It depends on what the state of the majority of components is. If they look weathered, have heat rash or condensation damage- then everything is gutted. Some amps are constructed so that it's difficult to unravel the leads. I don't mess with those unless it's just a few caps and resistors. Doing a 100% rebuild with the manufacturer's original circuit is feasable too.
I wonder how many hours it took to assemble something like a Fisher 500C?