From a quick search, Ubuntu LTS releases are supported for 5 years as a baseline, and Ubuntu Pro goes up to 12 years. RHEL releases are supported for 10 years.
I'm guessing it's similar with SUSE and other "business" distros.
Ubuntu Pro and RHEL are both 10 years for their standard lifecycle, with optional add-ons to go longer. Ubuntu's is called "Legacy Support" to get an extra 2 years, RHEL's is called "Extended Life-cycle Support" to get an extra 3-4 years.
