Apple has not provided a date for this requirement. Xcode does not apply the quarantine flags to executables that it builds, so they would be exempt from any Gatekeeper checks anyway. They don't even need to be signed, let alone notarized.
I'm not sure what you mean by "own installer". If you are using a DMG or a package maker archive, then there are some instructions and forum knowledge about how to notarize those installers. If you have some kind of custom executable, then you would need to notarize that executable. I have seen some mention of command-line tools and integration into CI build processes, but I haven't paid attentiont to them. You can search the documentation and the forums.
What john daniel said plus…
And is there any planned timeline when this notarization becomes mandatory?
There’s been no public announcement of such a timeline.
I seem to have missed the memo here. Sorry. See haikeeba’s post below.
Is there some instruction on how to best integrate this into a (CI) build process?
There is indeed. For an overview of this, I recommend that you watch WWDC 2018 Session 702 Your Apps and the Future of macOS Security.
For written documentation, check out:
These three documents cover most of the cases, but if you run into some other problem you can ask here for help (or open a DTS tech support incident for formal support).
Share and Enjoy
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
Apple has announced that notarization will be required in the near future, and some changes are coming Spring 2019. We recommend you begin integrating this process and let us know if you have any problems.
"Starting spring of 2019, macOS Mojave will more prominently highlight notarization status. In an upcoming macOS release, Gatekeeper will require Developer ID–signed software to be notarized by Apple."