well.... depending how long you plan to veg, your lighting, and other conditions it can vary.
my personal experience. from seedling... I veg for about 30 days or until the plants are 1.5ft tall. Then I throw them into flowering. after about 3-7 days you can start to see the sex on mine. but again it can be longer depending on the light you use, so this is just personal experience. now I clone, so I never have males to have to figure out the sex from anyways.