Purple lilacs, scientifically known as Syringa vulgaris, typically reach a mature height between 8 to 15 feet. However, with optimal conditions and care, they can potentially grow up to 20 feet tall. The growth rate and height can also vary depending on the specific variety of the purple lilac. It's important to note that these plants thrive best in well-drained soil and full sun exposure. Regular pruning after the blooming season can help maintain a manageable height and promote better flowering for the following year.