towtoe:
Text Messages are stored in a Database format, and not as individual files. It's unlikely that they will ever be "overwritten" in the sense you ask, but may well remain in the database tables either forever, or until such regular purge mechanism clears them out on a schedule.
So if you deleted messages, you can try two methods: You can use iTunes to retrieve deleted text messages on iPhone; You can try recover from iTunes with iPhone Data Recovery.

Yes, this way is very helpful for me. Thx!