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It's a case of habitable zones = chances of life = very good (but not a certainty)....I'd rate it at 90%. If the conditions for life, even as we know it are met, then chances are it's there. Look at our own planet....life got started here very early, before the LHB and even by then it was fairly advanced microbial life, so it was evolving even before then. It may have taken a long time to go beyond those stages, but that doesn't mean the same thing will happen elsewhere. Life might get off early on a planet and evolve much faster than here, or it may still be pea soup even after 5 billion years. Actually, the harsher the conditions, the more likely evolution will drive the appearance of advanced life forms earlier, so long as it's not overwhelmingly harsh. You could say diversity in adversity. Our little neck of the universe may have been a bit too cushy for life, for all we know.
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