Anytime now. You cut off all the leaves but leave 3'' or so of stalk on the plant. Shake off all the soil and store in a frost free place.

I'd personally leave them in the ground for as long as you can. A slight frost will only blacken and kill the foliage, i always lift them after this has happened in order to give those tubers as long as possible in the ground. The tubers only really get affected with severe, deep frosts.

I would follow Kildermories advice except once cut off, leave for at least 2 weeks before lifting. This will encourage eyes to form for next years cuttings.
If you follow badfish, again, once blackened, cut off and leave for 2 weeks.
At this time of year, tubers will be as good as they are going to be.

The ground is really wet after the storms, so I have begun to lift mine to dry out. With the lower temperatures leaving them till the first frost runs the risk of damaging the tubers.