Apple is good at offering at least some small update for devices it currently sells, recently they proved that they count that time from the date a device is released and not by specs only. If Apple doesn't introduce a new product BEFORE iOS 8 is completely finished all A5 chip devices will get an update (there's just too many of them to completely abandon at once - iPhone 4S, iPad 2/3, iPad Mini), this could change if a refreshed upgraded iPad Air comes out for example in May/June with iOS 7.3 and Apple stops selling their older devices.
So iPhone 4S & iPad 2 will probably get a last micro update because they were both released in 2011 and offer the same hardware but don't expect much new features or significant changes (since iPhone 4S was released a few months later than iPad 2 Siri is included while the tablet doesn't have it). We need to count the time from the system's number it debuted - iPhone 4S started with iOS 5 so it got a full update next year with iOS 6, then came iOS 7 and it got a partial update, in 2014 it'll get a symbolic last update in iOS 8. That means your purchased device is on a solid guaranteed 2 year cycle + 1 additional year of support for those that bought the device later and will continue to use it.