Completely depends on your week/stepping.
The best thing to do is keep lowering it until prime 95 errors out in under a minute. That will then be your absolute minimum. Then slowly increase it until you've found your desired stability.
In my experience with A64s I've found that 24hour prime stable is around 0.1v or 0.125v vabove the minimum.
As a big guess though, I'd say anything between 1.2v and 1.3v