people do this to help transition the plant into the flowering stage. You dont have to do it as just switching to 12/12 will intiate flowering without any darkness before, but some people beleave that it will help speed up the flowering process. usually 24 hours is what people do
It depends on your plants. The extra darkness will cause them to stretch. If they're really bushy, you can go 36 hours... if they're already leggy don't do it at all. Depending on your plants, you can find a happy medium. Normally, 24 hours is used. Any darkness past 12 hours increases the amount of time Pr is in the plants system, triggering flowering... An extra 24 hours at the beginning can shave a week off your overall flowering time, if you have bushy plants to start with... If you have leggy plants, 24 hours extra darkness could cut your overall yield by 20%... -Loki