Retrieving phone call duration on iOS?
Within the UI (user interface) itself you can only view the history of the past 100 calls, irrespective of whether those calls were all made within the past day or past month etc. However, this is an inconvenient way to view your call history if it's something you want to analyse, especially if it's on a regular basis.

For something a lot more convenient, such as viewing your call history in a spreadsheet or text file, you can use 3rd party software such as iExplorer.

With iExplorer you can access, view and export this data, either directly from your iPhone or from an iTunes backup of your iPhone. This is important because:


the option of access this data from an iTunes backup means you can go over the call history of past iPhones you've owned (i.e. if you have Time Machine backups or you manually keep old iPhone backups)
being able to view your call history in a text or csv file is so much easier to analyse


If you're interested, you can read How to Export Your iPhone Call History.

NOTES:


I am not affiliated in any way with the iExplorer software.
Not all functions are available without a purchase, however you can view your call history without purchasing it (but you won't be able to export the data)