Id bore over just enough to get out the pitting, whether that be +.25mm, +.75mm etc.
That way there is always the option later to overbore again if the needs be.
That being said in pure size terms, I think i've seen someone go atleast 5mm over....not certain though.
[edit] That's with new oversize pistons...which I assume you are talking about? If your talking about just honing to remove pitting and keeping the same pistons, well, im not sure.