Maybe two. After that much time passes commercial vehicles will be
available. So NASA should try to get it right and go out on a high
note: start with a LOX-kerosene flyback booster for Shuttle (one booster
replacing both SRBs), and then a replacement for the Shuttle orbiter (a
vertical lander which also serves as space tug and CRV, with a
7-passenger module which swaps out for a cargo pallet).

Or they can just keep doing what they're doing, and that will fill
enough time for the senior people to retire before it all falls apart.
(Anyone remember the United States Railroad Administration? They were
big in their day...)