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

https://web-wiki.win/index.php/Is_a_20-Year_Guarantee_Normal_for_Single_Ply_Roofing%3F

Single ply roof guarantees usually cover 15-20 years, but the actual roof can often last 25-40 years if cared for properly. Guarantees mainly cover material faults, not wear and tear or damage from poor maintenance