Welcome to iMore. Nobody could possibly answer that for you. People still use the iPhone 3GS. the 5s was introduced 3 years a go and is still being sold by Apple. Apple may not support a phone, but that does not make them unusable. I would hope to get at least 5-6 years out of it, if not more for what you spend on them. Truly a tough question to answer. I think you would need a crystal ball.
As mentioned above no one truly knows but my Mom is still using a like new iPhone 3 with no issues so it's how well you take care of it will ultimately play a role in its life span
Well, for me, hopefully 2 years, that's until the new model comes out. So if it last me 2 years I am happy as a pig in sh!t.
But I know of many people still using much older models, if you take care of your baby, it will last you a long time
I think it depends on what you're referring to. If you're purely talking from a hardware perspective then the answer solely depends on you as the user. If you're talking software support from Apple then I think 4-5 years, if I'm not mistaken.