Typically they lay the spring after they were hatched, so they're normally about 7 months or older when they start laying most likely.
The incubation period for guineas can range anywhere from 25 days to 28 days. Most websites will tell you 28, but for almost every hatch I've done they started pipping on day 25 (both in an incubator and under a hen). The humidity when hatching in an incubator can be kept about the same for chickens during the growing, but at hatch it must be raised very high. Pretty much as high as you can get it since guinea eggs do have very hard shells. Since you're hatching under a hen, humidity won't be a problem.
Mine don't use nest boxes, they lay on the coop floor or in the run until they find a nest. They free-range, so they nest in tall grass, shrubs/bushes, berry bushes, under wood..Generally they don't like nesting in the woods, they prefer a field. And my guineas have laid behind a piece of wood in the coop before, just have never set there.
Since you just started the broody on eggs tonight (If I read that correctly), then you could take the chicken eggs away for anywheres from 4-7 days and then give them (or fresh ones) back to the hen. That way the chicken eggs and the guinea egg would all hopefully hatch at the same time. Although it's still fairly early for guineas to be laying, and it is your guinea's first egg, so it may not be fertile.
