Pretty much until apps refuse to install, or work properly. That could anywhere between 2 to 5+ years. Ideally, I would recommend upgrading once your device stops receiving security updates. But I’ve seen people still using iPhone 6’s for example.

Until it stops working but I wouldn’t because it wouldn’t have the security updates

Like in a year or a year and a half, at most.

There is no way of knowing , as soon as a developer updates an app and changes the dependancies to a newer operating system version you are toast

Until the apps that you want/need don't work any more