I think you need to be using metric definitions to track this. Check out the module Metrics > Definitions.
Here is one that I think would be similar to what you want
This is from Product Docs
- Field value duration: This type of metric measures the duration of time from when the value of the specified field is set until it is changed. A Field value duration metric can optionally specify a script. The script can either return a duration value or set the answer variable to false to stop processing the metric. For example, the baseline incident metrics stop calculating duration when the Active field of an incident is set to false. The script can also carry out any other action such as closing the duration of other metrics defined on the same record. For more information, see Sample field value duration script.
Check out this article and the link to the field value duration script
