As Tead mentioned, it depends on your growing area and what you personally want out of your plants.
This plant is ready to flower once it is sexually mature. If grown from seed, many strains are mature in 3-4 weeks, if they are wanted to be kept small. If you are growing from clones, they are already sexually mature. You can keep either in veg as long as you'd like though, until they reach 1/3 to 1/2 your estimated final size.
Some strains will show you that they are sexually mature by producing pre-flowers. If you keep an eye out, these will tell you when you're ready and/or if you have any males. Other strains need to enter a 12/12 light cycle (or similar) to show sex.