The answer is none. No tolerance for corrosion.
But, Ok so lets talk realistic, not new motor desires/specs. You have to evaluate youre tolerance for risk.
What youve got is not optimal, but not in and of itself is a killer if compressions are good. That is to say the motor wont self destruct just because of this specifically, not now and not ever really. It may eventually start eating up oil if its not doing it already, or pollute the oil quicker than it should. To be explicit, a motor can run fine with zero static compression, will even run ok w/o piston rings. But it'll drink oil, make lots of blow-by and obviously wont make the kind of torque/power it should. Clearly noone would want that extreme for an airplane - obviously.
But what the cylinder rust in all cyls does is make the entire motor suspect to other items that must be looked at. Most importantly, is to get a look at is the cam for a def go/no go. Any pitting on the cam and its a rebuilder for sure. Then to: Does the motor have oil analysis history? Then to a current oil change, oil analysis of that sample, and cutting the filter.
If it has a CS prop, I'd pull that for sure and look at the sludge for a warm fuzzy. Thats a clear historical story on the motor, like a core sample. If you got rust particles in that, its been all through the motor, and is imbedded in the crank bearings, its a rebuilder IMHO.
If everything checks out ok except for the cylinder walls, I'd prob buy it, negotiate for a lower price and change out the cylinders anyway.....or if I felt ok about it, at some point when compressions, blow by or oil use indicates it. Prob wouldnt open up the bottom tho.
Of course, caveats right? Not telling you what to do, and this just my .02; just food for thought.