I'm speaking in general but if it depends on the vocal how can I find out by ear?
For all high pass and low pass filters for any instrument or voice:
Start with the filter turned on but at the end of its range (as low as possible for high pass, as high as possible for low pass). Listen to the track either soloed or with the music, depending on why you are filtering it. Slowly raise (lower) the filter until it makes the track sound bad and overly filtered. Then lower (raise) it slightly until it just stops sounding bad or wrong. Done.
How do you know when it sounds bad/wrong/too much? Vision and experience. It’s a combination of having a vision or plan for how you want the track to sound and the experience to know what that plan sounds like in each element of it.
If you feel like you don’t have enough of a plan or enough experience, then just give it your best shot and you can always go back and change it later as you mix. Doing it even when you aren’t sure exactly what you’re doing is how you get the experience.