I would leave the fleece on all winter. The fleece can be removed in the Spring when the weather warms up and the plants begin to grow. It can then be put back in place during the Spring on days when a frost is forecast.
What type of plants are you using the fleece to protect? I use fleece to cover crops such as broad beans and onions in the Kitchen Garden and on Bananas and Tree Ferns in my own garden at home. I leave the fleece in place all winter long in all of these situations.
If you are protecting specimen plants such as Tree Ferns or Olive trees, our Fleece Plant Jackets are particularly useful as they are very easy to remove and replace during the Spring. They are also made from a warmer fleece depending on the type of plant you are protecting.
I hope this helps you, but please email back to me if you have any further queries.