Before the release of Windows 10 1709 it was rather complicated, the answer is a minimum of 12 months support, according to Michael Niehaus, Director of Product Marketing for Windows at Microsoft:
Each Windows release, for example 1511, has a finite support time frame. This is at least 12 months, but it could be more based on the fact that we’ll always support at least two CBB releases in the market at all times, when the third one is declared, the first one drops from support. For Pro and preceding SKUs, you can specify that you want to defer upgrades, which causes new feature upgrades to not be installed until they have been declared CBB. (For the Home SKU, you can’t do this, so new feature upgrades happen automatically ...