Schmitt Trigger hysteresis
The correct value of hysteresis voltage depends entirely on your application, and you haven't told us anything about that.

Basically, you should consider three factors:


What is the minimum voltage that should look like a "high" input to
the comparator?
What is the maximum voltage that should look like a "low" input to the comparator?
What is the maximum voltage of the noise on my signal?


You probably want to make sure that the hysteresis voltage is greater than your noise voltage to prevent false triggering. Hopefully, the magnitude of the noise is less than the difference between your input high and low levels. If so, then set the hysteresis trigger points at the high and low levels you specified.