Text messages on an iPhone are generally stored indefinitely, though it depends on your service provider. Some service providers, such as AT&T, will store messages for up to twenty days, while others like Verizon may store them indefinitely.
You can also back up your text messages to your computer or cloud storage to save them for longer periods. It’s important to note that even if the messages are deleted from your iPhone, they may still be stored on the service provider’s servers.
If you need to save your text messages for longer periods, you may want to consider signing up for an online messaging service such as Google Messages.