not sure what you are asking but see if this will answer your question scrog is usually done with female clones as the sex is predetermined so there is no waste of time training male plants, a screen is placed over the plant most commonly 8-12 inches above the pots or hydro system or soil bed whichever you are using, this height gives you adequete space to be able to reach under the screen for training or trimming any excess growth once the plant just penetrates the screen training begins while screen height can be set wherever you wish, increasing it above 12 inches will increase veg time but might actually be of benefit if attempting to scrog from seed as you should be able to sex the plant before training if a higher screen height is used.
and for the second part of your question it is usually done in both veg and flower the goal is to have a full screen by the time the plants quite growing after a few weeks into flowering. if you wait to late to switch to flowering you can easily overfill the screen if you switch to flowering to early your screen wont be filled completly wich is still usually better than overfilled.
you can reduce veg time by running more plants and keeping a short screen height by using one plant per sq ft of screen and kepping the screen height to a minimum you can actually do most of your training after switching to flowering, versus running less plants where you would have to do more training during veg as each plant will have more screen area it has to fill.