This was implemented in Discourse Chatbot as a weekly quota system in March '23 and has since been expanded so you can define quotas for three different collections of user Groups. E.g. Paying members get a higher quota.
Users who breach their weekly quota are shown a polite message (which costs you nothing).
Moreover, the admins can be alerted when a quota is breached.
I did PM you about it in response to one of your prior Posts on this topic, but you didn’t respond 
Perhaps you are hosted and don’t have access to my plugin?
tbh, with the advent of GPT 4o-mini the costs for decent bot conversations have plummeted.
btw, Discourse Chatbot is now used by at least one business for front line customer support, so you can be sure it is stable and effective.