Figure out how tall you want them to grow. They will gain another 50%-100% in height after forcing or it might double in height! So trigger when the plants are small or you waste time in growth. 2 ft. tall max.
All depends on the amount of yield you want, and the room that you have. You can start at 12 inches (plant size) and as said it can double in size from start to finish of flowering. So do the math and see what you want.