I've always just used the old double the size measurement. Once put into flowering it normally doubles in height.

I.e. If its 2 feet tall after 6 weeks of veg and then put into flower it should roughly only grow to around 4 foot.
MarijuanaMike Reviewed by MarijuanaMike on . When to go from 18/6 to 12/12? I've identified my 3 plants as all females( I ordered female seeds), they are about 9 inches tall and appx. 4 weeks old. When should I go to 12/12? Rating: 5