One of the biggest topics of debate in the weed growing community, is exactly when to flower your plants. I am of the school of thought that your plant will tell you when it's ready to flower. Some people will always do it after a certain amount of weeks no matter the size of the plant. Other people will only begin to flower their plant when it reaches a specific height. I don't believe there is any right or wrong answer, as all of the methods work, but I prefer to listen to the plant. If your plant isn't ready to flower yet, I suggest you go back and read this guide on how to grow marijuana indoors.
During the plant's growth, I constantly monitor the height of the plant and write it down in a journal. I do this very every plant, and every grow I do. This gives me a good data set to refer back to in future grows so I can see what I have done wrong or right, or when a certain milestone is about to happen.
Using this journal around the 6th week of the plant's growth, I take a look back at all of my growth data. You will see a sharp bell curve form if you were to plot out the data-points. However, you don't need to go to this length unless you want to. You can simply monitor the plants grow for a sudden drop in gain per day. You will know it when you see it, and at that point, you should flower your plants.
As I have said, there is no wrong or right way to decide when to flower your plants. But I find it is always better to listen to the plant.