Usually OS's get supported for at least 10 years from the date their released, and sometimes even longer if certain sectors take a unique liking to them. But in the case of vanilla Windows 8, support and security updates ends today, January 12th 2016. So upgrade unless you want to be vulnerable.
It seems that because Microsoft released Windows 8.1 as a separate operating system, support for the previous OS will end. If you upgrade to 8.1, then you'll extend your OS support until January 10th 2023. Downgrading (or upgrading, depending on your point of view) to Windows 7 gives you security updates until January 14th 2020, and of course Windows 10 is available and quite the nice OS thus far.
Windows 8 is perhaps one of the finest mistakes that Microsoft has ever made. They were shooting for something nearly completely different that missed the mark while helping to drive the innovation necessary to reach Windows 10 and the more fluid update system that's been implemented.