There's no absolute value in which it's stable. It depends on the end result when the processor was manufactured
The only way to test if the voltage setting is stable is to start with something that's below the maximum voltage allowed (the interwebs tells me ~1.4-1.425V is the safe maximum but do your own research), see if the computer boots, then run a CPU stress test like Prime95. If it doesn't crash, it's stable. If it crashes, you need more voltage.