Stock debian is 5 years with their LTS project, but they have a paid "ELTS" project that adds an additional 5 years. So 5 years for free, 10 total years as a paid support option.
https://wiki.debian.org/DebianReleases
And that is why RHEL is so valuable in enterprise.
When you see people still running PHP 5, or Python 2, and not for tiny little nonprofits either... there are large sums of money being thrown over the wall to support that.