Until they start decelerating, which is immediately. They’ll also feel it fairly rapidly, I think.
https://en.wikipedia.org/wiki/Reduced-gravity_aircraft#Opera... (emphasis added):
“The sensation of weightlessness is achieved by reducing thrust and lowering the nose to maintain a neutral, or "zero lift", configuration such that the aircraft follows a ballistic trajectory, with engine thrust exactly compensating for drag”
Until they start decelerating, which is immediately. They’ll also feel it fairly rapidly, I think.
https://en.wikipedia.org/wiki/Reduced-gravity_aircraft#Opera... (emphasis added):
“The sensation of weightlessness is achieved by reducing thrust and lowering the nose to maintain a neutral, or "zero lift", configuration such that the aircraft follows a ballistic trajectory, with engine thrust exactly compensating for drag”