As a general guideline for boas I use their age, add a year, and that's the weekly interval between feedings. Once they hit four week intervals I keep them on that for life.
I may adjust up or down based on body condition, or if I think their last meal was a little lighter or heavier than normal, or if a neonate started as a picky eater and was a slow grower I'll keep it on weekly feedings for a bit longer.