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

https://daisydustmaid.com/overlay-vs-full-replacement-for-a-single-ply-roof-when-does-overlay-make-sense/

Single ply roof guarantees typically cover 15-20 years, but the actual roof can last 25-40 years if looked after properly. Guarantees cover material defects, not wear and tear or poor maintenance