I would say at least 1-2 years.
A4 chip
iPhone 4 (30-pin) [512MB]
iPod Touch 4th gen (30-pin) [256MB]
A5 chip
iPhone 4S (30-pin) [512MB]
iPad 2 (30-pin) [512MB]
iPad Mini (Lightning) [512MB]
iPod Touch 5th gen (Lightning) [512MB]
A6 chip
iPhone 5 (Lightning) [1GB]
iPad 4th gen (Lightning) [1GB]
As you can see, the iOS devices that Apple are selling mostly have A5 chips. This year they might drop off support for A4 as they would be no longer selling the iPhone 4 anymore (iPhone 5S, iPhone 5, and iPhone 4S).
My guess is when the iPad Mini with Retina display comes out, it will replace the iPad 2's spot at $399.
As far as app support, my iPad 1 is still supported. I think iOS 4.3 and 5 are the highest iOS needed to support. Using all of those apps is a different story, so you might not be able to run the latest apps in a couple of years.
A4 chip
iPhone 4 (30-pin) [512MB]
iPod Touch 4th gen (30-pin) [256MB]
A5 chip
iPhone 4S (30-pin) [512MB]
iPad 2 (30-pin) [512MB]
iPad Mini (Lightning) [512MB]
iPod Touch 5th gen (Lightning) [512MB]
A6 chip
iPhone 5 (Lightning) [1GB]
iPad 4th gen (Lightning) [1GB]
As you can see, the iOS devices that Apple are selling mostly have A5 chips. This year they might drop off support for A4 as they would be no longer selling the iPhone 4 anymore (iPhone 5S, iPhone 5, and iPhone 4S).
My guess is when the iPad Mini with Retina display comes out, it will replace the iPad 2's spot at $399.
As far as app support, my iPad 1 is still supported. I think iOS 4.3 and 5 are the highest iOS needed to support. Using all of those apps is a different story, so you might not be able to run the latest apps in a couple of years.