Debian 10 "Buster" Should Be Out Around Mid-2019, Debian 12 Is "Bookworm"
The Debian Release Team is currently planning for a transition freeze on 12 January 2019, a soft-freeze on 12 February 2019, and a full freeze around 12 March 2019. With that said, they are thinking the official Debian 10.0 "Buster" release will happen around the middle of next year.
Beyond that, for Debian 11 "Bullseye" meanwhile they are hoping to introduce more automated quality assurance (QA) testing with continuous integration, auto packaging tests, etc. Based on past release timing, Debian 11.0 will likely be out in 2021.
Further out, they have decided that Debian 12 will be codenamed "Bookworm". Obviously it will be several more years before seeing Debian 11 let alone Debian 12: likely we'll see this Debian update premiere in late 2023 unless there is a shift before then in their release cadence. Debian codenames continued to be derived from characters off the Toy Story film franchise. Bookworm appeared in Toy Story 3:
The release team updates were posted today to debian-devel-announce.