My rule of thumb is I expect a year of life for every $150-200 I spend on it. 

The iPod Touch I purchased in 2009 is still going strong; its only function is to be my alarm clock as it is forever stuck on iOS 5 and I have long moved on from the Apple eco system.

“Years of use, which are based on first owners, are assumed to be four years for OS X and tvOS devices and three years for iOS and watchOS devices,” Apple said

But from my personal experience

iPhone 4 - Don't know. I bought my iPhone 5 before the 4 gave out

iPhone 5 - 4 years - then it had intermittent problems including not charging.

iPhone 7 - 3 months and then the display died. The replacement has been going strong since with no complaints.

kharris:[snip] “Years of use, which are based on first owners, are assumed to be four years for OS X and tvOS devices and three years for iOS and watchOS devices,” Apple said

Which suggests Apple's figures are very conservative. Four years for a desktop Mac is a far shorter period of time than I would expect them to last.

I've only owned one iOS device, but within 2 years upgrades had pretty much crippled it.  Jail-breaking restored performance and gave it another 2 years before the battery died.

I find that they seem to treat iphones the best, in terms of longevity, compared to other ios devices. By the time ipads are released they are 6 months behind in terms of the chip they are using. ipods are treated worse, as the latest version is still using an A8 chip, and apparently it is isn't as fast as the chip in the iphone 6, which is some 3 years old. So when devices stop being supported by ios, you generally get 6 month less ios updates with an ipad, then you do with the iphone, as the new version of ios is usually released around the same time as the new phone model.  So to get the best value out of an iphone, you have to buy it as soon as it is released, or buy it later on a a good discount (which is rare). Although the iphone SE seems to be discounted quite a bit, pity it doesn't have 3D touch though. My main reason for liking iphones and ios, is mainly app support and reliability, which I find a lot better than my android experiences, and gerenally I have found the build quality and reliability far better. So due to the timelines, an iphone should last longer in terms of software support, than other ios devices, and that maybe one reason why iphones are so pricey compared to an ipad, which is quite affordable for the tech that is in it. 

I think 3-4, unless you don't treat it well or are unlucky

MikeAqua:

 

I've only owned one iOS device, but within 2 years upgrades had pretty much crippled it.  Jail-breaking restored performance and gave it another 2 years before the battery died.

 

Did you try just restoring without Jailbreaking? What did you do after jailbreaking to improve performance?