Single ply roof guarantees often last 15-20 years, but the real service life...

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

Single ply roof guarantees often last 15-20 years, but the real service life can stretch to 25-40 years with proper care. Guarantees cover material faults, not wear and tear or poor maintenance