Typically, based on my personal experience, I'll expect between 5-8 years out of a Mac.
To dive deeper though you can see my answer to a previous question, concerning fixing older Macs, that explains Apple's three life cycles when dealing with Macs.
Basically Apple supports Macs for a minimum of 5 years, though often longer, from a software standpoint. Third party software vendors (Adobe, Microsoft, etc) typically follow this time frame as well.
On the hardware side of things Apple considers Macs age 0-5 to be current, meaning replacement parts and services are readily available if needed. When a Mac is between 5 - 7 years old Apple considers it vintage, meaning they stop actively producing replacement parts and software support might start to trickle off. When a Mac is over 7 years old it hits Obsolete status in Apple's system. Obsolete Macs often have a hard time working with modern software, parts are scarce and more often than not unavailable from Apple and all support services from Apple cease.
Specific model Macs rarely deviate from these timelines, though as you found the early Mac Pro is one example in regards to software support. This was one of those odd instances where it simply lacked the required architecture for 64 bit operating systems. It's unlikely to see such a paradigm shift going forward, at least for the next 6-7 years.
Hopefully that explains it a bit better.
Source - I worked in the Apple break/fix ecosystem for 6 years, for both an AASP and as a Mac Genius.