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

https://blogfreely.net/rothesqlmw/why-does-the-cheapest-flat-roof-quote-often-fail-early

Single ply roof guarantees usually cover 15-20 years, but the actual service life can stretch to 25-40 years with the right care. The difference comes down to day-to-day wear, weather, and maintenance