Not sure if 8.1 will remain on the store until the actual date stated in the policy as it is still considered, by Microsoft, as Service Pack for windows 8
Windows 8.1 falls under the same lifecycle policy as Windows 8, and will reach end of Mainstream Support on January 9, 2018, and end of Extended Support on January 10, 2023. With the General Availability of Windows 8.1, customers on Windows 8 have 2 years, until January 12, 2016, to move to Windows 8.1 in order to remain supported.
Source
So it could be as soon as
January 12, 2016 if the date of the Service pack expiry is used or as late as
January 10, 2023 if the official end of life is used