How often does the iPhone X require you to change your passcode?

Unless you are using a device profile through a company or educational institution, there aren’t any

requirements or guidelines governing how long you can keep a specific passcode on your iOS device. You can use the same passcode indefinitely if you choose to do so.

On its own, never. You can setup a passcode and never change it. The iPhone X should not require a passcode change after one has been set up.

If you have a Microsoft Exchange email account, normally provided by corporations or education institutions to employees or staff, it may require a periodic passcode change on the iPhone, but its entirely up to the Exchange server how often it requires it.