Count from first formation of flowers. That could be a week or two after flip, conditions and strain dependent. A mature plant will usually kick into flower quicker than one from seed forced into 12/12.
Its really not about days or weeks though as the others have said. Its about watching your plant and as Monkeygrinder said, the Calyxes will swell or fill out, become nice and dence, Pistols shrink back and trichs change from clear to cloudy to amber. Depends on what you like ass far as those go. If it goes too long you can literally see the amber shine with your own eyes. Get a good trich scope, or even a cheap on. They look amazing when you look close up.
Commercial growers (not in the usa) love the seedbanks time scales because they just don't care about the quality, they arnt gona dry it properly anyway so why would they care, its about the money for them.
Look at what you guys n girls are doing in America these days.. Its amazing how far standards have come since the laws have changed. Leaders in the market especially in quality. One day I hope my country can go the same way.
C.W