Timeline for an 18.x release?

If compiling, the OpenWrt Development Branch is 18.x, whereas the LEDE Development Branch is 17.x.

As to your overarching question about a stable 18.x build, probably not in the immediate future due to the amount of devices OpenWrt supports.

  • I'm not sure if OpenWrt devs & maintainers are of the same belief as other Opensource OS devs/maintainers, but generally speaking, most devs/maintainers of OSes generally aren't a fan of being asked when the next release will be done, due to getting asked a lot, in conjunction with the complexities of new stable releases for all supported devices,
    • I'm not saying this is the position of OpenWrt devs/maintainers