Compared to RHEL/CentOS the Debian release cycle is actually very short, with support for an older version ending just one year after the release of a new stable version. So in average you will only have support for about 3 years for each release, compared to at least 10 years for RHEL. That's why there has been some discussion about a possible Debian LTS release: http://lwn.net/Articles/565007/