You also have to remember that you're feeding mice. They are a much leaner prey item than rats so your snake won't grow as quickly as a snake of the same age that's being fed rats.
We purchased an 8 month old yellow belly that was a 'mouser' as per her breeder that weighed 230grams. She's never turned down a rat for us in the 4 months we've had her, eats once a week like clockwork a live prey item that's 10-15% of her body weight and has more than doubled her size. She's now just under 700g.
As long as your BP is getting fed an appropriately for its size, it will continue to slowly grow. Keep in mind they don't all grow at the same rate. Personally, I have all my snakes on rats. Even little Buggsie, so if someone tells you they have to be a certain size before you can switch them, kindly tell them to stuff it.
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