How to know the duration of a "non-WIP" RegeRequest and the Merge?

As Method & Tool, I would like to know the duration between a MergeRequest without WIP dans the corresponding Merge.

How to do it ?

  • via the web interface

  • via the API

    If it’s not yet possible :
    Is it possible to add in gitlab API, the argument With/Without WIP Data for MergeRequest ?