A few remarks:

• For now the users are just informed about the upcoming expiration of certificates used in the Sign/Encrypt dialog. There is no button to act or get further information what to do about it.
• Expiration of issuer certificates are ignored. If a leaf certificate gets invalid as soon as any certificate in the issuer chain expires, then it may make more sense to treat this as expiration of the leaf certificate since that's effectively what happens. On the other hand, if the expiration of certificates in the issuer chain have no effect on the validity of the leaf certificate (because at the time the leaf certificate was certified the chain was valid), then, in my opinion, it makes little sense to bother the users with the expiration of chain certificates.
• I took over the default values that are also used by KMail and that seem to be the recommended default by SPHINX (according to the comments for the settings in KMail).
• I decided to save/load the thresholds from a shared configuration file (kleo-expirycheckerrc), but to keep the setting whether to show expiry notifications as per-application setting.

Works. The only drawback is: for the message to be displayed in the "for others" part of the encryption dialog you have to click in the next line before it is displayed.
If you click on sign/encrypt directly, you won't see the warning. At least if you select the recipient by starting to type and the selecting from the dropdown.

The corresponding issue for it to disappear: if I delete the recipient again the warning goes away only after clicking in the next line.

In contrast: if you use the button on the right side for selection, the warning for the chosen recipient is shown immediately.

And I cannot find the kleo-expirycheckerrc you mentioned, it's probably optional and to be located in %APPDATA% kleopatra?
Instead there ist a configuration option in the appearance tab of Kleos configuration menu, which works, too.

Mmh, why did this go into its own rc file. I guess its because its in libkleo and not in Kleo itself. I have to check how this works with my registry code. I doubt that this will work out of the box but its important for us that our settings can be set through the windows registry.

@aheinecke As I wrote "the thresholds should be shared by all applications". Therefore (and because the code is in libkleo), using kleopatrarc wasn't an option. Or is the question why I didn't use libkleopatrarc? One advantage of using a separate file is that watching for relevant changes by other applications is much easier resp. that one doesn't get change notifications for unrelated settings.

In T6330#170382, @ebo wrote:

[...] The only drawback is: for the message to be displayed in the "for others" part of the encryption dialog you have to click in the next line before it is displayed.
If you click on sign/encrypt directly, you won't see the warning. At least if you select the recipient by starting to type and the selecting from the dropdown.

The warning is shown either when the users accept their input with Return or when the navigate to another UI element (e.g. with Tab). This is done to avoid distraction by too much jumping of the UI while the user is interacting with the input field.