Like Daybreaker said, it depends on the snake. Mine goes into a mini hibernation for about 2 weeks, doesn't eat, doesn't move, just sits and sits and then when it's ready it just does its thing.
Within a week or so of going blue I generaly notice they all of a sudden they clear up like they've already shed, but no shed in the tub. Once they clear it's usually within a day or so for me.
Re: How soon after the first signs will a BP shed?
Mine are 24-48 hours after they turn clear. However, all mine shed in their tubs, so I can never catch them in the act. The only one I have caught on camera is one of my corns who sheds around 8pm at night.