It takes about 21-28 days for Guppies to produce fry. During this time, i would make sure that there are plenty of spaces with hiding spots and plenty of plant material. A great plant to use is hornwort. It floats on top and gives a place for the fry to hide. Do not use those breeder boxes, they always stress out the females and in long run have them die or cause the fry to be aborted.
jim