It's hard to put a number on it (e.g. do people with eyesight problems or headaches actually know it's their display?) and it may also be influenced by other factors. I have used phones with 240Hz PWM dimming for years and haven't had an issue. However, I quickly get eyestrain when looking at e.g. a Samsung S26 Ultra. It's probably the combination of 240Hz PWM, FRC for emulating 10-bit on an 8-bit panel, and maybe the two types of pixels (narrow and wide, for the privacy display) that makes it painful for me.

I can also imagine that lower frequency PWM dimming can be more problematic on a display that covers a larger portion of your sight. Many people also probably use their laptop/computer screen for a longer time without looking away (aside from phone addicts).