How to calculate Stage durations for Business process flows?

Hello,

I have a requirement from my customer to calculate the duration of sales process stages each user is going through.
eg for all leads qualified as opportunities in 2017: how long were they in Qualify, Discovery, Execution stages etc by owner.

I figured out that achieving this requirement was pretty straight forward to implement in CRM v8.1 or earlier. (below link)
https://community.dynamics.com/crm/b/nakedcrm/archive/2017/06/06/how-can-i-work-out-the-time-spent-in-each-stage-of-a-business-process

However from CRM v8.2 onwards, there has been data model changes in the way the business process works internally...it seems that each BPF spawns a new custom entity..

I was planning to add a custom entity which could track the stage start and end times using a workflow.
However to track the time duration for each stage, I need to know when to trigger my workflow which should ideally happen whenever the BPF starts or stage change happens.

Any ideas, suggestions welcome!