Something like 15 years and 7 major version releases. Photoshop CS6 is 13.0, where Photoshop 6.0 was, well, 6.0!
Photoshop has had two very different, somewhat overlapping, version numbering systems in its history.
The original Version 1 to version 7 lasted from c.1990 to 2002. It still continues in the background under the new version numbering scheme.
Photoshop version 6 shipped in 2000.
From 2003 onwards Adobe changed the marketing name of the product line to "Creative Suite" and product numbering was reset to CS (known as "CS1" by implication).
Thus we now have
CS (also known as CS1; version 8 under the original system)
CS2 (version 9)
CS3 (version 10)
CS4 (version 11)
CS5 (version 12)
CS6 (version 13) - shipped in 2012
Thus there is more than a decade of new and enhanced features (and some removed) between version 6 and CS6.
Note that on John's link are only listed the major features...
There are usually hundreds of changes between each version.
On the Macintosh side, there has been major rewrites, from Classic to OS-X, CodeWarrior CFM to Mach-O, from Power PC to Intel code, from Carbon to Cocoa...
Small technical point: not rewrites, but refactoring.
In computer science, rewrites have a long history of failure.
Refactoring means replacing subsystems, or putting abstractions in place to make large changes easier to manage.
That's an interesting observation, Chris. All the things I've ever rewritten came out better than the junkware I was tasked with fixing.
The history of rewrites (at least for large software) is not good.