You can change to 12/12 whenever you feel is necassary. You have to keep in mind though, whatever the height of your plant is before you switch to 12/12, will double, even triple in size by the time you're done flowering. So if your plant is 5inches high, the finishing height will be approx. 15-25inches in height. There is no right time and wrong time to do it, you make up your mind about how much space you have, and time you have.

I flower my plants when they are about 10inches high(almost a ft). They finish usually around 2.5ft(depends on the strain)

The light cycle you are on now, 19hrs on you said? When you want to switch to flowering, you go from 19, straight 12/12. Don't gradually change, just change it in one go. From 19hrs on, to 12hrs off, to 12hrs on, etc...get it?