Start palpating her and find out if she has follicles and what size they are. She's not ready if they are the size of BBs. If they are gobstopper(candy) size, then try to start pairing. Some girls go quickly, some take their dear sweet time. I barely cooled mine(85 hot-80 cool) for 16 hours a day, then for the remainder, it was back to normal(95 hot, 84 cool). The girls all took about 3-5 months from first breeding to ovulation to egg laying.

Depends on the female though..

And why would a 300g baby be getting 65-75g weaned rats?