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

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 roof can last 25-40 years if well cared for. The guarantee mainly covers manufacturing faults, not wear and tear or damage from poor maintenance