Are you considering raising them as feeders? If so, in my experience it is not worth the time and effort. If it's for a pet, they are very cool! I'm not sure how long it takes from egg to adult. I've always either purchased nymphs or adults. My one inch nymphs took 3-4 weeks to grow to about 3 inches.
Since most species are very cannibalistic, they need to be separated into individual containers or they will eat each other. It's the feeding/cleaning/spritzing each individual cage that will take a lot of your time. Additionally, you must consider the cost of feeders (and they go through a lot when they are young). Breeding them to keep the cycle going is something I have not tried. You must be careful to make sure both mantids have eaten and usually introduce the male when the female is in the middle of a meal.
I think most people consider mantids to be bad feeders since there is not a large return for the effort. I like them as an occasional treat for my chameleon and am willing to pay for a few large nymphs from time to time.
They are however, very cool pets! Keeping one or two is not very difficult at all and they are fascinating to watch.