Well “all good” is subjective and depends on lots of factors, but generally I would say amongst consenting adults, as long as you’re open and honest, then there isn’t a moral dilemma caused by a relationship having a known expiration date.

The best time to tell them is as soon as possible. The longer you wait, the more morally dubious it becomes imo. I think there can be some exceptions to that based on things like personal crises of the person being broken up with, but in general I would say that it is not your job to protect another adult from information they may find upsetting. It is patronizing to do this, and by contrast, the most respectful thing you can do is be politely and compassionately honest with them as soon as you can.

Don’t be rude about it, but if you’re situation is something along the lines of “I enjoy this relationship but I know that it will not work for me long term” then you should tell them that as nicely but as clearly as possible, and as soon as you can so that they aren’t being lead on.

If they accept that, and you both choose to continue until the expiration date, then there’s nothing wrong with that. If instead your partner decided that means the relationship is no longer worth their time and effort, that’s a perfectly reasonable response and you should do your best to part ways amicably, or transition your relationship into whatever form is most desirable to both of you under those new terms, ie friends with benefits, just friends, or simply exes of whatever level of estrangement.

That’s my advice at least. Again, it’s more generalized as I don’t know the specifics of the relationship.

But generally speaking, the ethics of relationships comes down to consent, and that is only possible when both parties are informed.