In the Conversation area of a record, when I send a message to someone (@John Doe) and the person has not responded to my message within 24 to 48 hours; I would like to have an automation set up to send them a notification/reminder that a reply/response is needed.
This would be very helpful. 
Thank you.
You can set up an automation in monday.com using a combination of @mention detection and time-based triggers, but since native automations don’t track replies, a workaround is to use a Status column like “Waiting for Reply.” Then, create an automation like:
“When status is ‘Waiting for Reply’ and time has passed (e.g., 2 days), notify person assigned.”
This way, if no one updates the status (i.e., no reply), the reminder is triggered. 
Sorry its not exactly what you wanted