Not scientific by any means but I've been feeding approx 60g rats to my BP and she has gained 30-40g each feeding. So to answer your question, around 50-60% or more.
I can't answer your exact question because my snakes will eat a few meals before defecating. So I can't be accurate with the exact weigh ins after each meal and poop.
But my hatchling and sub-adult snakes gain about 80-100grams a month with weekly feedings of appropriately sized rats.