The only way to know for sure with a plant how long the various stages will last is to grow clones from a mother plant that has already demonstrated this phenotype. Seeds will also provide you with particular phenotypes (as advertised) but with seeds there is always a chance that the recessive genes can resurface in any individual seed. Seeds from the same plant can demonstrate different characteristics. Again the only way to know for sure is to work with clones.
Also you can always force a plant into flower. Some growing techniques have you start seedlings off at 12/12 so that they will go straight into flower. Even though plants will continue to grow in size in flower once they truly begin flowering the plant stops growing new nodes and puts its energy in growing all the bud sites it currently has. Let a plant veg longer it gets bigger and adds more nodes/budding sites. The longer you veg the more bud you’ll potentially get.
Ultimately what experienced growers do is find a strain that works best for their grow environment. Then you tailor your technique to fit your grow and your plant phenotype. SOG, training and topping, are ways you can maximize you grow to fit your area and technique. Choosing how long to veg depends on how you set up your grow area and technique.
