Resolution time is the time between a customer first getting in touch and their issue being fully solved and the conversation closed.
To calculate it, subtract the time the conversation opened from the time it was closed, then average across conversations for the period. If a conversation is reopened, decide whether you measure to the final close, and be consistent.
It is different from first response time. You can reply in five minutes and still take a week to solve a damaged item claim if you need photos, a courier claim and a replacement. Customers care about both.
The biggest delays in a shop are usually waiting on someone else: the customer, the courier or a colleague who can approve a refund. Hobson proposes the order action, such as a replacement order or refund, with the drafted reply, so it can be approved in the same step. See Shopify actions and the damaged item template.