Speaking to pdiak@redhat.com  and pliurh, we decided it will be beneficial to report the time taken for migration as well. This will help us to avoid having to check timestamps every time we are testing migration, potentially avoid mis-calculating manually and may be also used in our regression tests possibly?
 
Snippet from oc get clusterversion -o json|jq .items[0].status.history after upgrade:
"completionTime": "2024-04-10T19:55:32Z", "image": "registry.ci.openshift.org/ocp/release:4.15.0-0.nightly-2024-04-07-120427", "startedTime": "2024-04-10T18:48:26Z"