Dicor, the major supplier of "rubber" roofs for RVs, warranties the material for 10 years. They usually last longer unless they have been scrubbed too aggressively or get torn by a tree branch. Full replacement is $3000-$4500, but most people get by just painting on an elastomeric roof coating (e.g. Kool Seal). You can do that yourself for a few hundred dollars and a full day of work.