Standard Report: Time between Actions Merged

We have an internal SLA with our sales organization.

Our team needs to understand the time between actions (origination date of an idea, time to the ‘next’ action). By way of example, how do we report on the time between the default needs review to the next action (like future consideration) and so on.

How do we create this or is there a standard report we are overlooking? What are the specific fields we should poll to build this report?

This should NOT be a manual report.