its depends on type of cpu.
older cpu usage more voltage like pentium series processors used 2.9 V to 3.3 V.
but nower days most of cpu are created with idea in mind that it can be attached with laptop,phone and tablet or surface too. so idealy it should be between 1.25v-1.5v if its more than 3.3V than there will be chances of heat sink so cpu will not work more than 30 min.
hope that helps.