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

https://www.slideserve.com/gwyneyxcez/why-do-roof-problems-show-up-after-two-winters

Single ply roof guarantees usually cover 15-20 years, but the actual roof can last 25-40 years if looked after properly. The guarantee mainly covers manufacturing defects, not wear and tear or damage from poor maintenance