Fast Flowering you grow like photoperiods, veg as long as you would like.
They're supposed to finish earlier, if the photoperiod variant is 12 weeks then the Fast version could be 8-10 weeks.
I've seen growers that had them take just as long though.

Start at the lower end of the nutrient dose and read the plant, if it needs more or less then it will tell you.
That's what I do every grow regardless of type, ease my way into it.

fast flowering here and you are right these are on a 7 week veg now in week 5 of flower for 12 weeks total they were 19/6 during veg then 12/12 , i grew some fast flowering before this they took like 12 weeks to flower