Determining the right female to breed can be very difficult, as you want to make sure that your breeding pairs will produce healthy kits. Every chin is different so I don't think its fair to say that they should be X many of months and X grams. I think its more important to look at the overall chin, as standards develop more quickly than mutations, and can enter breeding earlier.

Another thing to look at is the pelvic spacing, which should be about the size of a quarter or an adults thumb wide. To check the spacing, feel for the pelvic opening which is a soft spot, and is located right under the tail.

Do not assume that this means you should breed an underweight female or a very young female just because her pelvic spacing is big enough, as that is not healthy either. If I had to give a general time frame, I would say in between 8 months and 1 year and over 550 grams.