Thatch can provide a very durable and handsome roof. In the U.K., where thatching has a rich history, there are instances of a water reed roof lasting over 100 years. This example is exceptional. I would estimate a typical reed roof to last 50 plus years. The quality of the water reed, the skill in thatching, and the environment that the roof is exposed to would all contribute to the longevity or lack thereof.