While I agree with rangerdanger that it's best to wait for the plant to naturally show that it's mature enough to flower (alternating internodes) one should know that the once you put a plant under a flowering regime it will start to alternate internodes automatically. This basically means that you can force flower which forces maturity at any size. But....small plants results in small yields. The bigger the plant the bigger the harvest. That means that the longer you veg the better but allow enough head room for the plant to triple in height. Head room is the area between the top of the medium to the bottom of the light at it's highest setting. If you have 6 feet then start flower at 2 feet. Rule of thumbs are different for other alternate forms of growing like scrog or LST.