At what point (HU) do you quit playing flops and focus on AI only?

[CardSharpCook]

SNG. You've gotten to heads-up against an opponent. The blinds just went up to X. At X, do you never call PF, do you only go AI or fold? What is X? Can there always be some play? Are there some hands you might just call with or just min-raise with? Does your opponent matter?

There was a brief debate about this in the MTT forum in which my answer to this question was widely disputed. I'm trying to fix a hole in my HU game. If you've read that thread in MTT, please don't respond here - I'd really like non-biased SNG pros to answer.

Thanks for helping.

CSC