Orbit (EDIT: more like escape velocity)
Well, kind of. Gas is similar to anything in space: if it's in orbit, it won't come down until the atmosphere or solar radiation decays it's trajectory enough. But no matter what, it will settle onto the top of the atmosphere if it stays in orbit. If it escapes the Earth's gravitational pull then it will never come down.
It's not necessarily a matter of altitude, but more of the orbital trajectory of the particles.
As for the crossover value, it depends on the engine, namely its exhaust velocity and plume dispersion, as well as the ascent profile of the rocket. So the answer is that there isn't really any set threshold that will define, for all rockets on LP-1/LOX or any other fuel combination, when 50% of the exhaust will fall back to Earth and 50% will not. It is entirely dependent on the particular rocket launch.
EDIT:
Let me rethink my answer...
Let's assume a Merlin 1D from SpaceX as an example, running on kerosene and oxygen.
In a vacuum, Merlin 1D has a specific impulse of 311 seconds, so our exhaust velocity $v_e=g_0\times I_{sp}$, so about 3,050 m/s.
If the rocket is at the end of its orbital insertion burn into LEO, then it will be traveling at orbital velocity, 7,800 m/s. If the exhaust leaves the engine the other way at 3,050 m/s, then the final velocity of the exhaust would be 4750 m/s, well under orbital velocity. It will be recaptured by the atmosphere.
If the exhaust is to leave the Earth forever, then it must be traveling away from Earth at escape velocity, 11,200 m/s, and then some to account for other forces. That means that the rocket itself must be traveling at escape velocity plus the exhaust velocity: 14,250 m/s. The rocket must be well on its way out of the Earth's gravitational influence if it is to stay in space.
Again, this leads to the same answer that there is no practical 50%. It is more of a binary returns-to-earth or doesn't-return-to-earth outcome. Altitude is not important, it is velocity and direction of the exhaust that matters.