If it were me, I'd get out of the milk jug and into a legitimate pot for a couple weeks before considering flowering it out. I've never grown in one before, but I feel like it may not go too well, later in flower. I'd get into a 3 gallon pot, get back on a veg cycle, and let it grow into the new pot for maybe two weeks. Then get back on 12/12.
But to answer your original question, it can take up to 2 weeks to see preflowers after switching. It usually doesn't, but it can. And if they've got too much N, this may delay the onset of flowering as well.