Welcome to the forum!
Your snake sounds lovely.

I have a 9 month old snake that is 1400 grams. I have an older 9 month old at 500 grams.
All snakes grow at their own pace. There isn't really a set average or pace.

If you don't know the weight, then as long as she has good body condition(full, round, well muscled), I wouldn't worry about her size in regards to her age.

You can feed your snake every 7-10 days.

Personally, I feed my hatchlings every 4. Then bump them to a 7 day schedule once they reach 500 grams. But these are intended/current breeders. Pet adult BPs do well on a 7-10 day schedule.

And I agree with what the others said about your temps being too low.