Tasty, you'll probably need to wait at least 4 weeks or so, but really it depends on what you are growing. I think that the cloche helps established plants grow faster and better in cold weather and survive frosts that they might not otherwise survive, but for most crops if it wouldn't germinate or survive at all without the cloche it's probably too early to grow even inside a cloche. Perhaps someone knows better than me.

You might do well to get a copy of _The Maritime Northwest Garden Guide_ by Seattle Tilth, it will give you good guidelines on when to start growing stuff.

I'd agree that it's too early for seeds, even in a cold frame. It's simply too dark and cold. I don't even think about putting anything into my cold frame until mid-march. And those will be sprouted indoors.