It's really hard to estimate.. especially since you can't judge age on a wild-caught ray.
From what I've seen, the larger species of rays take longer to mature.. Rays like Tiger and Castexi would probably take a very long to become sexually mature, especially the females. Keep in mind they have much larger pups, and many more at a time, so they will need more birthing capacity!
Smaller Rays like Reticulata have been known to have fully rolled claspers at 6" and less than 1 yr.. and some have bred at around that 1 year age!
I think 2 years is a good estimate, especially for the commonplace rays like Motoros.. not sure on Hystrix.
You might find this link interesting, and I will post it up as well;
http://journal.nafo.int/35/charvet-almeida/22-charvet-almeida.html