How long does Lagerstroemia indica grow together?

About five years. If four crape myrtle trees are put together, if the seedlings are small and straight and close to each other, they will basically grow together and stick together into a tree after five years, but they will still see the traces of integration. If the seedlings are big and crooked, they can't be in close contact, only the well-contacted parts will stick together, but eventually they will grow into a whole and become a tree.