so how much weight should my hognose gain each month
That totally depends on the sex, age, and what you’re feeding. As long as your snake is healthy and eating well and the right size food you shouldn’t have anything to worry about. If you think your snake isn’t gaining enough weight we need a lot more information.
I 100% agree with what @erie-herps is saying. It truly depends on your animal, as every animal is different and grows differently.