When the heating goes on for an hour at tea time.
Actually Tilly, the notion that garden spiders come in for a warm is a common misconception.
Garden spiders don't come into houses as a rule - what you are seeing is male house spiders who have been born and living in your house - and now it's mating time and they are looking for females.
For that reason, you should not put house spiders outside when you find them - they are unlikely to survive.