Canonical releases new Ubuntu versions every six months, but most of those are then supported for 9 months only. If you want long term support, you need to install the LTS (Long Term Support) version released every two years such as Ubuntu 16.04 or Ubuntu 18.04 which are supported (for free) for 5 years. If for whatever reasons you cannot or do not want to update after 5 years, the system will not get any security fixes, unless you purchase a Ubuntu Advantage subscription with Extended Security Maintenance, which allows you for example to run a fully supported Ubuntu 12.04 (Server) until April 2020 (8-year support).
But this will change for the better, as ZDNet reports that Mark Shuttleworth told the audience at a keynote at the OpenStack Summit in Berlin that Ubuntu 18.04 support will be extended from 5 years to 10 years
“In part because of the very long time horizons in some of industries like financial services and telecommunications but also from IoT where manufacturing lines for example are being deployed that will be in production for at least a decade.
It’s unclear whether the extended support will be available to Ubuntu Server only, or Ubuntu Desktop will also benefit from the longer support period. I could not find any confirmation that the extended support would be provided for free, so the extended support may be a paid option, but instead of just 3 years extra like for Ubuntu 12.04, that could be 5 years extra instead.
The 10 minutes keynote is embedded below.