How do I configure ARCHIVELOG mode retention period?

Hi, I want to enable ARCHIVELOG mode on my database.

My goal is to allow for recovery from these logs from a weekly cold backup we perform.  Ideally I'd like to have the ARCHIVELOG files go back 2-4 weeks, depending on size. 

I've enabled ARCHIVELOG mode on a test database with very little activity 2 days ago. Our database has about 213G total in data files and it's already accumulated about 3.4GB in archivelog files with the database mostly sitting idle.

Additionally - how do I configure the frequency period?  The archivelog files appear to be written every 2 hours or so.   As it stands we do nightly database exports with datapump, so in a disaster recovery scenario we'd lose up to a day of data.  I'd like to try and get this down to 1hr or less.