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.
If you're really worried about this, just use Top-Lock or Flex-Top nuts (all-metal nuts with deformed threads). I'm pretty sure Ma Mopar used something like that.
