I learned awhile back about clipping asters from a neighbor who's own always seemed to be completely under her control. I wouldn't have ever known if I hadn't spied her cutting them.
Each year the time to consider clipping might be different. This year, it's quite early. I've clipped my taller asters, those above 16 inches, back to about 13-14 inches. No matter the height, consider reducing clipping back by 15-20 percent. Doing this will promote more flowering, as clipping can with so many plants, because you cut the dominant growth tips (apical meristem), allowing many side stems to grow. Aside from bushier plants with more flowering buds, it also keeps the plants a bit shorter, and less likely to flop as far when heavy with flowers in the fall.
You can be graceful or rough about the clipping. I tend toward the middle.
All the asters that are quick to get tall get this treatment. This includes the New England asters, the perennial sunflowers, and even goldenrod -Solidago.
This aster, at 8 inches tall, isn't grown enough for clipping. Maybe in late May or early June. Here in New York City, I wouldn't clip after late June. At that late date, you will weaken the plant, invite disease, and limit formation of flowering buds. So to be safe, clip early when the plant reaches 16 inches or more.