Soon.
Make sure you have lots of floating plants in the tank for her and the fry.
Don't move her into a breeding net/ trap, just leave her in the main tank. Moving her now could induce a premature birth.
And no big water changes now either, not until 1 week after she has given birth.
You can let plastic plants float instead of putting them in the substrate. It does the same thing.
Try not to move the female back into a tank with males for a week after she has given birth. This will give her a chance to recover before being harassed by males. And try not to handle her with a net for a week after she has given birth. Use a net to push her towards a container and scoop her out in some water. If she is well fed she should ignore the babies
Wondering if my female guppy is pregnant and what to do if she is