Jump 4 frames, bair 4 frames, so at least 8.
you're airborne frame 4, so it overlaps with the bair's startup if you do it as fast as possible
so 7, actually
/nitpick
Sheik's u-smash is alright.
You kill off the top with it as a combo finisher and toss it into combos to extend them where applicable. Sometimes it's decent after jab resets or if you predict / guess a non-tech / tech stand of a space animal / Falcon.
It's usable on most getup attacks (which is pretty much only relevant vs. space animals and Falcon 'cuz otherwise regrab leads to better stuff). If you think Fox or Falco is going to waveshine through you, it's also good.
You can also gamble her invincibility timing (her arms are invincible during it) against their move / motion and try to score some KOs or combo extensions that way.
There's probably more but those are what I use it for... it's not always the best choice (or even a legitimate choice) because of its speed but it's got some really useful traits (invincibility, can be done out of shield, alternate knockbacks, etc.) so you can come up with good applications.