It is acceptable if someone mention it on their personal text or signature
No, it should be directly in the post, and not formatted to be unreadable.
2. How likely moderator take action (delete reply/thread or lock thread) only because there's no mention of AI usage if someone decide to make report?
I'd be strongly inclined to take some sort of action in all cases of undeclared AI usage, though in
some cases the action might just be a warning, if the post is otherwise very constructive.
3. What counts as only occasionally? Below 50% of all created post/thread in last month?
Not many people have been banned for excessive
declared AI yet, so I don't have enough experience to formulate more specific, reliable guidelines yet. It's at the moderator's discretion.
What I'd tend to ask is: if you were not using AI, how much less text would you be posting? If it'd be a reduction of more than 5-10% overall, then you're using AI excessively. For example, posting an AI's response to your 1-sentence prompt magnifies the amount of text you post by a lot, so you should do this only very rarely. But if you write a unique and on-topic 500-word post, give it to AI to clean up, and it produces ~500 words, then that counts against you a lot less, in my mind. (Though I wouldn't
encourage doing that.)