When is my guppy going to give birth?

I've own 8 guppies, and one of my females has fallen pregnant. I know she is pregnant due to her growing quite large and having a pitch black gravid spot. About half an hour ago I saw a long thin white substance hanging out of her anal area. I know that this is not an internal parasite as she has no symptoms and neither do the other fish. Is this white string a sign she is giving birth? And if so, approximately how long will it be til she goes into labour?