It's got more to do with plant maturity than height. If you flower before it reaches maturity your harvest will be seriously affected. If you're starting from seed you need to give it 8 weeks before you start flowering and will probably have to top it and maybe lst it to keep it's size managable unless you've got no height restrictions. If it's a clone then it will be at least a month old when the cutting was taken so another 4 weeks of veg and it will probably be ready to go into flowering.

Good luck!