Always had end of Jan as the cut off date and no later and that has always worked for me, though I have seen people clip later and not have a problem - thought the idea of a full clip isn't a bad one now its warmer - but that is me being lazy and hating to go through the removal of the winter coat. My last clip for my boy meant I took everything off. Unfortunately my old girl needs her woolies so looking to take a small bib out.. and deal with the winter coat normally (yeuch!)
I clip at the start of winter, usualy leaving head and legs. Then at the end of Jan/beginning of Feb i do a full clip to get rid of lines. Works for me.
I always clip into March if necessary as they tend to get cat hairs which are like coconut fibre to groom!! I had once thought end of Feb was last date but now do this and have no problems with lovely summer coats. The other thing is that I also take the lot off as being lazy I can't stand grooming masses of hair off both nag and my coats! I recommended this to a die hard friend and now she agrees and takes the lot off too!