Snakes are incredibly efficient in regards to waste. Keeping in mind defecation is just that. Waste.
Anything that isn't used, is passed out as a toxin and expelled as waste. A growing snake has little waste.
It's not uncommon for a snake to defecate every 6 - 8 weeks while growing (depending on age and size), and even then some can go longer, and often it's timed with a shed.
If your worried about impaction, there are other more obvious signs than lack of poop. The vent area, will be hard, and non-malleable like it normally should be, when you run your finger along it. It's fairly obvious for even those that aren't experienced in it.