Single ply roof guarantees usually 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 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