Two years, give or take a few weeks, is roughly the time required to prepare a new stable release since Sarge in 2005 (source:Â http://en.wikipedia.org/wiki/Debian#Release_timeline). A Spring release has also been a pretty stable pattern during that time.
Debian often has a reputation for being very slow to release (in terms of cadence), or for freezes to hold up development for an excessive length of time, but the current pattern does lend itself well to two attributes: predictability and reliability (in terms of high-quality releases). It’s true that this means shipping with slightly older versions of major software, but you can sleep well knowing they’re thoroughly tested – and that’s really important in Debian’s core market, which tends to be highly-available services rather than desktop machines.
There’s always backports.