Look at the specs that were supplied with your monitor. The maximum safe refresh rate changes with resolution.
Or go to the manufacturers website and look for your monitor. They will list the supported refresh rates like this:
In VESA mode, which is what you're probably using
"X x Y" resolution / "X"Hz
For example
1024 x 768 / 87i, 60, 70, 75, 85Hz (119Hz max)
So that last number the "max" number is the maximum refresh rate supported at that resolution for your monitor. All the other numbers are the supported refresh rates, but that doesn't mean you can't use like 100Hz. You can because it is below the max.
Hope that helps