What is the end support date for Loki? Will Loki follow Ubuntu's support dates?
I guess it will have updates until next elementary OS release (0.5) but the backend (Ubuntu) will keep updating until EOL, which is IIRC in 2021
The elementary team typically only supports the latest release with feature updates, which I think is mainly a volunteer issue than anything else. The team tends to not set firm release dates and follows a "done when it's done" mentality. So now that Loki is released, there isn't a set date for Freya's end-of-life, but Freya-specific issues will likely not garner much attention. You can track the progress on the next release, Juno, here. That will likely be as close as it gets to Loki's end-of-life.
The OS is built atop Ubuntu "Long Term Support" releases, which also stop receiving feature support after about nine months but do receive major security fixes for five years (April 2021). The team pushes those security updates to former releases and will post in a public place if for some reason an older release cannot update (users would be expected to upgrade for security reasons at that time).