a couple days. Another guy said that if you put a brown paper bag just over a part of a branched out plant under 12/12 you can get just that part covered to show sex. he said it shows in a few days and at the most it took him was 2weeks.
No, you need to veg a plant 5-7 weeks before it will show preflowers. At 2 days your plant doesn't even have a true leaf yet, its barely broken soil, impossible to see preflowers. Preflowers are just the plants way of showing that it is sexually mature.