As policy,when is decided when the various KDE software is updated?
When there is a tested PR that updates the various components.
I believe there is no specific policy for kde. (My knowledge is limited though)
See Nixpkgs: flow from master to channel - #2 by ElvishJerricco for general channel update
Noting that this question has been answered already , I will add for anyone interested that there is currently a blocker to upgrading Plasma past 5.16 – see PR#71232 for details.
(p.s. As I know that packaging desktop environments isn’t an easy task, I’d like to also give a big thank-you to everyone involved in doing so for NixOS. Sincerely, an appreciative KDE user.)