with bagseed it is impossible to tell how long it will take. You may want to start your plants indoors under 24 hrs of light, you could do this with some compact flourescents or t5's or a cheap metal halide light. Get them nice and big and when you see pre-flowers or alternating nodes then transplant them outdoors to finish up.
Or you could just plant them outdoors, but with those short days it may take awhile, especially if you end up with sativa from your bagseed...which it is highly likely if your messing with Jamaican pot. You have to really watch your plants and make sure that they are not hermi's or else you will end up with a bunch of worthless seed.