You CAN SDC as many times as you need/want to. But, be aware that you don't HAVE to SDC more than ONCE in order to do what you need.

Your post makes it sound as if you feel that in order to SDC from an early flight to a later flight, you need to keep jumping to the 'next flight' every 2-3 hours all day long until you get on the 6 PM flight. Not the case.

To SDC from the 6 AM flight to the 6 PM flight, simply MISS the 6 AM flight (doesn't matter if you're checked in or not, as a courtesy, if i have checked in I call delta to have them uncheck me so the seat can be more easily reused.) Then, at 3 PM call in and SDC to the 6 PM flight. That's all...

When doing an early to late SDC, you always run the risk of an IROP wiping out availability on the later flight. So, check and double check availability (expert flyer is the most reliable too for this, but there are others.)

John