When you have the plant in 'veg' this means that you have more than 12 hours of light on it a day. Preferrably ~18 hours on and ~6 hours off. When you switch to 12 on and 12 off the plant starts to flower. The flowering process is what creates the buds.
You can't really go by the height because some plants are tall and some are not but I would say you could do it after its a month old. I could be wrong but thats what I think. I keep mine in the house on 18/6 for the first two months and then change it too 12/12 to flower.