Single ply roof guarantees usually cover 15-20 years, but many roofs last 25-40...

https://mega-wiki.win/index.php/Why_Do_Roof_Problems_Show_Up_After_Two_Winters%3F

Single ply roof guarantees usually cover 15-20 years, but many roofs last 25-40 years in practice. The difference comes down to things like weather, wear and tear, and maintenance