It usually takes them 3-4 months to become adults. I always tell people buying roaches its usually a 6-9 month process before you will start using them as feeders. Usually you buy nymphs because adults are hard to find. It takes 1-2 months depending on the size when you get them before they are breeding. Then it take your first few hundred babies 3-4 months to become adults an extablish a large colony. At this time you will have other 1-2 month old nymphs you can feed off, and hundreds of adults producing more babies. I personally seperate my adults into a new container every 90 days or so and let all the nymphs grow up. This way its easier to pick the nymphs you want for food without disturbing the adults everyday.
Good luck,
James