General guide is 2 winters.
I don't follow the 1500 g guide either. Some girls are sexually mature but don't ever get that large no matter how much they eat.
As long as the mature female has proportional weight to length, it should be fine to breed. For example: breeding a female that is 1200 g, but is short and fat is safer than breeding a skinny1800 g female. As long as the female has nice body condition and proper fat reserves, a lighter female can be bred safely.
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