Czevak had to wear a cryogenic suspension suit to preserve him, he was so aged- so that does show he's old.
I've got Grey Knights but not the next two in the trilogy- and I couldn't find that bit. Indeed, I found a bit that said he was "dismembered by cultists" and while Imperial medical knowledge was able to save him, from then on he needed an exoskeleton.
Though a Cardinal in Dark Heresy: Blood of Martyrs, while "well into their third century" so around 250-odd, plus or minus a few years- needs massive medical assistance to survive, he's so ancient. Maybe Cardinals move less than Inquisitors do.
So 300 does seem to be near the upper limit.