Single ply roof guarantees usually cover 15-20 years, but the actual roof can...

https://knoxuqsk688.cavandoragh.org/what-should-a-roof-survey-focus-on-for-an-older-single-ply-membrane

Single ply roof guarantees usually cover 15-20 years, but the actual roof can last 25-40 years with good care. Guarantees often focus on materials, not wear and tear or damage from poor maintenance