If I've got a female in her 3rd winter, the LOWEST weight I'll personally breed her is 1500 grams. I would prefer she's 2000, but that's just me.
I'm with Robin I'd like to let all my girls get to that 2000+/- gram range. But if you just find that you can't wait 1200-1500 is a good safe beginner range to start trying to breed at.
When you've got 10,000 people trying to do the same thing, why would you want to be number 10,001? ~ Mark Cuban