i don't yell commands, simply say them loud enough for the competitor to hear (sometimes, of course, i find i need to speak up a little).
as for standby; it's short, no stand-byyyyyyyyy....
as for the time after that command, it varies widely. anything between 1-4 secs, unless, as i'm watching the shooter, he/she starts to move, lean, creep, flinch, turn, edge, crouch or whatever else it is he/she thinks he needs to do.
but that's another matter.