The answer to the question asked is "when they are ready".
The real question that seems to be intended (or maybe I'm reading too much into it as a parent) is "when should I start worrying that my child isn't starting to recognize letters?", which may indicate some form of condition or learning disability that should be addressed as early as possible to give your child the best hope for a fruitful and enjoyable education.
The answer to that question is "when they start a focused course of study that requires letter recognition as a pre-requisite". Back in my day, preschool and pre-K were for kids from 3-5, with Kindergarten starting between 6-7. My daughter started her preschool just shy of 2.
Between the earlier onset of socialized education, and advanced in technology like the iPhone/iPad, kids are able to start toying with self-educational means much earlier. I wasn't all that shocked a few weeks back when I got up from my laptop in mid-Quora-answer, and came back a few minutes to notice that the middle of my answer was filled with my daughter's name repeatedly spelled out correctly – and she hasn't turned three yet.
Would I be worried if she couldn't identify the whole alphabet by now? No. I had peers in my kindergarten class who couldn't recognize basic words at 6, and most of them grew up to be very successful people. I'd say just try to enjoy the process as much as possible, try not to worry, and if they start really seeming to struggle while TRYING to learn something, bring it up to their teacher(s), or another parent of their peer/friend group. It's not a competition, after all.