there are a couple stages. first there is seedling, then veg, then flower, then harvest.
there are 3 types of plants. Sativa, indica, ruderalis.
most plants take approximately 1 - 2 weeks after you change the lighting period to go from vegetative to flowering (this does not include ruderalis)
Ruderalis has a set cycle and will flower when it feels like. (usually 14-28 days after seedling) it will also flower regardless of lighting cycle.
The basic lighting cycles are 24/0 (ruderalis) 18/6 (vegetative Sat/Ind) 12/12 (flowering Sat/Ind)
some say it is best to slowly transition from vegetative to flowering thereby reducing stress on the plant ex. day 1 of switch 18/6 day 3 17/7 day 5 16/8 day 7 15/9 day 9 14/10 etc etc etc.....
this is said to ease it into flowering because plants have "light cycle memory" and gives them the ability to produce and store the proper amount of starches and stuff.
so after you switch it over from 18/6 to 12/12, about a week or two depending on strain, if it a ruderalis, 14-28 days usually. this also depends on growing methods and environment conditions.