Take a look in the Metric Definitions view at the "Incident State Duration" - first step is to make a copy of that for Requested Items and point to the field that you want to monitor.
Once that's configured, each time a Request changes State (or Stage, or whatever field you set it on) then it'll create an entry in the Metric Instance table. You should be able to build your report against that table, filtering for Definition = ReqItem State Duration (or whatever you end up calling the new metric definition) and you'll get the rows you want to report on. Then it's just a matter of tweaking the visualizations to see what you want to see.
Note that you're going to be getting a new row for each State that a ReqItem goes through, so depending on what you're reporting, you may end up needing to use the Count Distinct option in your report to count each ReqItem only once. Only for those things that require it though.
For example, if you want to report on all of the ReqItems that were in the Waiting state, then you may want to use Count Distinct depending on whether or not you want to include ALL the times a ReqItem was in Waiting, if it was in Waiting more than one time.
Hope that made sense!
