Your iPhone 14 is still perfectly fine - keep it as long as the battery doesn't get really weak or you notice apps starting to lag 🔋. That's the only real reason to switch. Yearly updates are marketing hype, not necessity. Most improvements are minimal and the 14's camera is already damn good.
What a lot of people miss: phone batteries do degrade over time, but that's no reason to get a new device. You can get the battery replaced relatively cheaply - at Apple or at third-party shops that do it. That might cost 50-80 euros and extends your device for years. Way cheaper than a new phone.
Just do what feels right: if your 14 is still running smoothly and the battery holds up, stick with it. 2-3 years is the norm, 4-5 years is also possible. The only important thing is that you do iOS updates regularly (relevant for security) and don't fall for the hype. Your girlfriend just likes spending money 😅