You cannot. There is nothing stored about when a post is added to a relationship field.
It might be possible but you’d need to build some way of recording this information yourself. For example you could create an acf/save_post action that fires before ACF has saved new values (priority < 10). In this action you can compare the old value using get_field() with the new value looking in $_POST[‘acf’] and then somehow record when new values are added.
And you’d need to figure out how to store that value show that it can be queried, and I thinking about that I can’t come up with anything that could be stored in a single query-able field.
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