Adding multiple servers to a zone will distribute requests between all servers. Requests are routed based on the client IP address and the number of active servers in the zone. A client IP will always be routed to the same server, as long as the list of active servers does not change. Each time a server is added or removed, the traffic is redistributed. Updating the settings for a server does not cause traffic to be redistributed.
There is no warm up or adjustment period associated with the load balancer. It will respond immediately even as demand increases or decreases.