For once an easy problem to solve! We needed to change the retention values for OperationsManagerDW (SCOM uses OperationsManager as the main store, this is the data warehouse) and it was surprisingly easy using the notes from Understanding and modifying Data Warehouse retention and grooming. Download a utility, unzip (no install), copy/paste the commands into Notepad and replace with the instance name, then execute the commands.

The values we had match the ones in the post, so I imagine those are defaults – I don’t know if the user is prompted to change them or not, or if it’s able to forecast usage.