how long should i flower for

i have a kush plant my first grow and i dont know what kind of strain is it the seedwas giving to me by a friend it looked like a watermelon whith black stripes on it i think is buubba kush how long should i flower for it is alred giving buds at 3 week into flower how much longer the buds are barely growing

Did you stop giving nitrogen already? if so id still give them some as that helps them grow more veg / buds. As far as when to harvest, cut that off soon and give them more (P) to finish up, they will tell you when, it will be much longer than 3 weeks. The hairs are the 1st sign they are getting close, they will turn reddish and die, than you can look at the crystals (trichomes), theres alot of info on this that will tell you exactly what they should look like at ripeness.