In playing around with this, I think it may not be possible to modify this default search folder. But you can create your own. Maybe you can change the shortcut to point to your new folder instead of the default.
Finder > Go > Recents > Action > Show Search Criteria
Change "Raw Query" to "Last Opened Date" and select something like Last 30 Days or whatever you want. Then save this search. It will show up in your sidebar but you'll have to play around with keyboard shortcuts to see if you can point the default shortcut to your new folder. I can't seem to modify or change the default Recents folder