They seem to flower when they want to really.
Of the 5 I've got going from the same seed stock, but very different parents, I had hairs forming on 2 at least long before flowering started. Those 2 had tons of hairs within a week. 2 other plants took a week before they started showing up. Then the last one, that's still got no brown hairs at 59 days, took over 2 weeks for hairs to even show up and another week and a half before it looked like the others did within a week or two.
And absolute worst case scenario... You got the wrong strain and instead of flowering in 10-12 weeks it'll be done in 8-10 weeks. Other than that there's nothing to fear from flowers showing up early unless it's happening in veg and veg has stopped which either means an autoflower strain or simply a strain that wants more hours of daylight.