It's kinda impossible to say for sure. I had one guppy who had just barely started to swell give birth at least a week before I thought she was due, and another who was swollen so large I wondered how she was able to swim for a full week before she gave birth. Both had healthy fry. It's a bit like human gestation... we know it should be x amount of time, but then sometimes it's a bit shorter or longer.
Watch her from the top down. If she starts to look very fat and has a little trouble swimming or is moving slower, that's usually a good sign. Sometimes they stop eating within the 12 hours before giving birth, too, but then again that doesn't always happen either.