What manufacturer? The life will depend on many factors. Most critical for nylon or similar plastics is UV exposure. I don't think there is much UV exposure under the car. Also, if you care to spend a little extra money, the Mil-Spec nylon insert locknuts can be re-used, because they use a better grade of nylon in them. Yes, some holding value diminishes with repeated use. But they are not going to fall off.
