Guppies give birth about once a month. Takes about 3 months for the babies to reach breeding age, maybe a little more, then another month for them to carry. So you're looking at 4-5 months between generations. Make sure you start off with quite a few females to one or two males, or you wont have enough fry.
Another thing to consider is that guppy fry are surface swimmers. If you want a better chance for your cray to get some fry, go with swordtails. They generally stay towards the bottom in my experience. Swordtails have a little longer between birthings (average 34 days) and take about 4 months to mature, so slightly longer generation times.
Depending on the type of cray, adult guppies arent out of the question either, which might throw some kinks into the plan. Also, feeding the fry for faster growth may also feed the cray. It may not opt to eat as many fry as you would like.