I'm not sure if how long they "should last" matches most home owners experiences. Looking at many consumer complaint websites showed many users with failure between 5 and 7 years.
Recommended Maintenance Didn't Save Me
You should always do the recommended maintenance, but in my case maintenance wasn't the problem. I have a 14.5 SEER, 2.5 Ton Carrier unit. The evaporator coil started leaking after 43 months (under 4 years). It was a small leak where the copper met the rusty metal housing. I used high quality filters and replaced them every 2-3 months. They never looked dirty but I still replaced them within the recommended window. It was under a service contract and had preventative maintenance done by a licensed tech from AC-Designs in Central FL every 6 months (twice a year).
The parts were under warranty but labor was not so it was a little over $600 to replace with a 90 day labor and 1 year parts warranty. The tech did note that my case was unusual and he's seen both extremes with some lasting the life of the system (15+ years).
One thing he mentioned on replacement is some replacements are aluminum based evaporator coils vs the copper in hopes of a longer life. I don't know which is better but I found this article about copper vs aluminum here.
Depending on the system, sometimes it makes more since to replace the whole system. Get quotes on each (preferably by 2 or more vendors) before you decide.
The department of energy has some other great info on AC systems here.