It was suggested to me that at 4 to 5 months i should start gently stripping and trimming my puppy. I took her to a nearby breeder at 6 months for him to show me how to go about trimming her as i had little idea, and he introduced her to the coat king. Her puppy fluff had already started to become loose in some areas, and so she was ready for it then; this may not be true for all puppies though. Also, I think it was a good thing for her to experience the clippers etc so young (if left to grow natural she has an enormous moustach, so that had to be clippered off) It's a good idea to get the pup used to such things early in their life, even if they dont really need much doing to their coat just so they get used to it. As far as I am aware, in showing, minor puppies can still have puppy coat, but after 9 months it would be expected that the coat was stripped and trimmed, so i guess that is another benchmark you could use.
But...I have little experience with cockers and trimming, and so someone with more experience may suggest otherwise.
sportsmonki