Back in 2009 when the Obama administration was considering producing a heavy lift kerosene engine there was talk of resurrecting the proposed, reusable RS-84 engine. This article from 2003 said it would take until 2007, 4 years, to produce it:
RS-84 Engine Passes Preliminary Design Milestone.
Huntsville – Jul 16, 2003
“The RS-84 is one of two competing efforts now under way to develop an alternative to conventional, hydrogen-fueled engine technologies. The RS-84 is a reusable, staged combustion rocket engine fueled by kerosene — a relatively low-maintenance fuel with high performance and high density, meaning it takes less fuel-tank volume to permit greater propulsive force than other technologies.”
Unfortunately it was cancelled in 2004 after the Ares V was decided upon. But if all the development materials and designs from then were saved and assuming there was actual 1 years further development up to 2004, then conceivably development could be restarted and completed in just 3 additional years.
In any case I’d like to see a study done to see how long and how much it would cost to complete its development.