Congrats! Heavily gravid female guppy. As for when, could be tomorrow, could be a week or more, it's really hard to tell. When she develops a square, boxy shape, that means it's close, 2-3 days usually, but not every female gets that "squared off" look every time. The live plants for her to hide in while she births and for the fry to hide in are good though!
I'd urge you not to move her to a breeder box or anything. Let her birth in the main tank. If you want to save fry from being eaten, check the tank early morning and late at night for fry -sneaking down in the am and checking with a torch before the fish are awake is a good way to spot and net the babies before the other fish wake up! Then you can net the fry and move them to a breeder box to safely rear them if you like. Once they're 2-3 weeks old, they're big enough to avoid being eaten (depending on other tank mates!) and can go back into the main tank to give them more space to grow.
Reason we don't recommend moving the female is that it's possible to injure her when she's heavily pregnant by netting and lifting her out of the water, and it's very stressful for her to be moved and confined in a tiny clear box, and stress is the worst thing for pregnancy and labour. Can result in aborting or losing the fry and potentially the mother.
You can add more live or fake floating plants if you want to provide more dense hiding space and give the fry a better chance of making it. Guppy fry instinctively head for the surface when born, and will gather in a big mass of dense plant matter if it's there. That can make a great baby nursery if you want to leave them to grow in the main tank too. Feed the adults on the other side of the tank, then small crushed up food directly to the fry in the plant matter, so they don't have to venture out for food.