1. When Ashigara is complaining about being single;
2. When she's insisting for marriage or at least a sexual favor; or
3. When the Admiral is ignoring her for other shipgirls, or simply ignoring her at all.
Note: A "Christmas cake" is a Japanese idiom for a woman over 25 who is still single.
Mind that this pool is co-related with the anime canon, not in-game canon.