The "Max" is whatever you can sustain without instability, or overheating.
I have seen a few 146's run solid for months on end 24/7 at 1.65v. When returned to more reasonable voltags they ran fine. I have also seen 165's run at 1.8v for torture testing for thre days of hell without a hitch; they also still ran fine at default voltages.
The point is that the maximum voltage is when the CPU dies. Anything other than that is not "Max". As far as safe? Has anyone done a comprehensive lifespan chart versus voltage for ANY CPU of andy manufacturer?? NO!!
Therefore, you are on your own as far as "Maximum safe" voltage. Any use of the CPU shortens it's lifespan, and the second that it is booted it is dying. Default voltage is killing he CPU, but slower than at 1.8v would. Will it live forever at default?? No... How long will it live at 1.6v?? Likely longer than you'd want the CPU to be in your rig. Who still uses a Pentium III for anything outside of internet browsing or in the office. It is worthless for most anything else where time means anything to you.
That being said, what the OP should have asked is "How long will my CPU live with elevated voltages?" Of course the answer is likely to be a question......