In about 2 years. By that time it will actually be stable and pretty much bug free. For me Debian is the only distro that provides a reliable experience.
Well I don't actually use KDE. I just am stating what I've noticed in the past. KDE doesn't have stability as a priority so using old versions is really the only way to not have bugs.
Though, I know of and have reported a ton of bugs "Resolved in Plasma 6", where backports would make no sense as these architectural changes where so big.