it's an automatic flower, so you only know when it goes into flower after the fact.
with an autoflower, not a photoperiod, you can safely assume the preflowers are a good indicator. When you see them, it might be 5-10 days after the fact. So, it'll always be a guess as to the exact day the plant went into flower. Figure 5-10 days befoer you see the first preflower. if they are tiny nubs but you can resolve what it is, obviously closer to 5 days.
those are fairly small but already showing some stigmata. So, i'd guess you entered flower 5-7 days ago.