If your looking at bagseed, the POTENTIAL, for male to female is as follows (this is just a guideline and not what you will experience):
10 seeds -> 8 to germ (get that white thing growing our of the seed), 8 germ'd seeds -> 2 or 3 female.

So basically from bagseed, you could get about a 10-30% chance of female, and you wont be able to tell until it goes into flower. When growing from seed, it should veg for somewhere around 8 weeks, but others go sooner, completely depends on when the plant is ready (read the sticky's on when to tell), thats why The_image_reaper said you would grow old. If you really want to only grow one plant (and if your first time grower, i would suggest getting more than one on this cause you WILL make mistakes and possibly kill them) is to talk to your "guy" and see if he can get you somewhere around 2-4 clones. This will give you females since the mother is female.

Hope that helps explain a little better....