Time spent at Capacity in Salesforce Chat Session Report

Total amount of time in seconds in which an agent’s queue was full.
This field would capture the value of the time (seconds) the agent is at full capacity which is 100%.
This field will be blank if the agents never reached 100% capacity.

Chat Session Reports - https://help.salesforce.com/articleView?id=reports_live_agent.htm&type=5
1. Create a Custom Report Type with Chat Session as primary object.

2. Add Time spent at Capacity to the report.