I would just say as soon as she begins to need it :) Some people say you should only do it at certain times, because it damages the coat, but I've never experienced that
Don't forget to substitute what she might lose heat-wise with a rug when needed
I'm in Alberta, and I'm clipping my guy this coming week sometime....he's indoors, but he sweats so much that I can't get him dry.....I think anytime before Jan 1st is good.....although I'm just being cautious in saying that, as I have never seen any problem with summer coat coming in and clipping.....but I've heard of issues.......
Its not so much when to start clipping as it is when to stop. I start clipping as soon as I see them getting fuzzy, but I will not clip after February. You don't want to clip/ruin the summer coat coming in.