How much testing to commerical aircraft manufacturers typically do on each new plane after it rolls off the production line?
Testing is performed for specific reasons.  As you noted testing for certification and testing production aircraft are not the same.  

Testing for certification is to establish that the design of the aircraft meets airworthiness standards. A guide is available in the form of AC 25-7C.  

Testing production aircraft is primarily a quality control issue and serves to prove that the aircraft is built to the drawings -- that it complies with the approved design.  It's only a small part of the process and is discussed in section 2.7 of AC 21-43A.  

As noted, in the AC a flight test is required (with one exception). Production testing will evolve over time as there is a constant effort to 1. ensure adequate test coverage to avoid quality 'escapes' and 2. to do it as efficiently as possible to minimize costs.   

One other test that is typically performed is an acceptance test. This is not regulatory. It is contractual and is performed with the participation of the customer.  Airlines won't accept delivery of an aircraft until they're satisfied it meets everything they're paying for. A friend of mine who worked for an airline's engineering group said it was about a 4 or 5 day process to checkout and accept delivery of a plane.