I'm busy cleaning up a lot of old data, layers, apps, maps, etc. in our ArcGIS Online content but I don't see the credit amount declining. Are credits returned when space is made available?
Think of your credits as a sort of subscription - you use up the credits performing certain functions and storing data. The credits used for storing data are calculated by how much data you are storing and for how long. They are consumed over time. They don't refer to available capacity (and therefore more credits don't become available if you delete items).
If you delete enough data your credit consumption should drop/slow, but you won't get the credits back.