If it takes the snake more than 15-20 minutes to ingest the meal from the time it has actually started consuming it, the food item is unnecessarily large. If the snake inhales the item in about 5 minutes or less, it's time to upsize.
Sounds like a good rule of thumb, though I do have a boa who would blow it out of the water - she keeps attempting to eat her meals from the wrong end.
For the OP, my BRB litter last year got bumped from a mouse fuzzy to a hopper weekly when they were two months old.