Response time calculations are based on difference between the agent response and the previous customer message. That means even within the same conversation, each reply (back to the customer) is calculated and factored into the average. The calculation also only takes place if previous message is from customer and in an "Unresolved" state. If either the previous message is from a staff or if the conversation is closed, Reamaze will stop calculate response times.
One more thing to keep in mind is that if there are two consecutive customer messages, the staff's response (and response time) is calculated based on the first customer message among the two and not on the second customer message.
If you need to wait a long time since the last customer reply to follow up, you can pause response times by adding an internal note or setting a reminder on the conversation.