Scoop already has this feature built in and the settings are in basically the same place in both v2 and v3.
Go to the settings (options for v2) menu > Scoop Database Settings and click on the Categories tab. You'll see a grid with all the category names and there is a column called expiry. This is the number of hours to keep stories for in each category.
Set to this zero if you don't want the category to expire.