The standard "target" for Debian releases, at least from the Lenny release and newer, is 2 years. I know that this had to be their policy in 2004 when Ubuntu started up because this is the base on which Ubuntu has based its "stable" LTS releases. They release them every 2 years and supported them for 3 (now 5). This was all on account of the Debian schedule of a about 2 year release cycle and one year support for Old Stable after the New Stable is released.
Ubuntu LTS is built on Debian testing. Regular releases are built on Sid.
It will be a damned long time before Debian 10 is released. Debian 9 is new. Debian 8 is still well within Debian Official support.
The so called "LTS" for a full 5 years of support is not at all an Official Debian Support. The additional support time support is provided by people that choose to do so. This is primarily supplied by Ubuntu paid devs that are paid to work Debian packages. As Debian is a non profit Canonical can then use their salaries as a charitable tax deduction. Certainly some of the many companies using Debian web servers also do the same thing.
I have tried the LTS version past the Official support by Debian and have not been at all impressed with Desktop support on it. The core system works fine but gui support is rather slow and behind times. Not something I recommend for PC users at all.