My mum has just got an iPhone 4S and it actually runs suprisingly well.
She should, it still has a good balance of power-per-pixel. Better than my iPad 2 or the iPad Mini 1. And obviously the expectations of what is good enough vary per person. That is why I kept saying "effective" life. To me that means the part of that device's life where the performance is as good or better than the day it was purchased. What is useful to each person is a personal judgement, but when a device is slower than when it was bought is obvious to everyone.
For example, I would hate to use an iPad 1 or 3 regularly, but there are people here who claim to do that all the time. I think part of that is selective ignorance- it feels fine until you play with a modern device and then you get a mean case of the wants. But the fact that some people have lower expectations is besides the point- Apple will run almost every iOS device to a version of the OS that runs slower than the one it came with. That is the downside of a one-way "support" system.
I'd hazard a guess far longer than any Android phone released in the same time period, regardless of how much RAM that Android device has.
I will take that bet, but then you might not know about the fact that as of Lollipop almost every Android device has had the major parts of the OS decoupled from the OS and shoved into the app store. Therefore any Lollipop device will be supported as long as that device can connect to the Play Store, which means all of them should still be getting some level of OS updates long after the 6 and 6+ are off the iOS treadmill.
You are going to say that is not fair though, and force me to compare an Apple to an orange (aka full OS updates). In that case I still say you might be wrong. I mean Google updated the 2012 Nexus to Android 5.1 Therefore the Nexus 6 might have a longer support life than the 6 or 6+, it certainly will have a longer effective life. Just because we don't have any 2011 Android around in 2015 doesn't mean we won't have 2014 Androids around in 2018. Android was in a different place in 2011.