Man, I don't know if I want to get involved in this discussion---its too depressing. [Heavy sigh] But, against my better judgement:
A buddy and I were whitewater kayakers, but were kinda stuck at the Class III 1/2 level. We got invited to meet some friends in ID for some bigwater Class IV-IV 1/2 boating.
We were scared. We had the chops, but our Eskimo roll was suspect: we could do it, but panic sometimes caused us to flail our rolls and take swims---sometimes nasty, once or twice terrifying. However, trying to improve our roll while in the shizz was only going to lead to trouser-soiling. :gross:
So we disced down.
We went to Shilshole Bay and Lake Union every evening for a week, and did 50 rolls and paddled a mile or two each time. We were using the same skills we'd be using under combat situations, but without the danger.
Neither of us swam in the two weeks in ID. In fact, I started doing first descents, and big[35'] falls, and it wasn't till 9 months later I had a minor swim on a piddly little log[heh].
I think DDing should resemble what
Oklahyzer suggests---a cleaning of form, so when you need to depend on it, its good. How long? If you're a new thrower, it may take a few weeks/months[can't remember how long it took me to learn to throw a 69g Regular Frisbee---Chris'sakes, I was 7yo]. If you already have experience, a week or two might do the trick.
But every now and then we take a trip on a Class II river, and just play with small waves and eddies, rolls, and not being intimidated by the water we like to paddle---scary water. We call it hairboatin'.