She'll poop when she's ready. Cleo went almost four weeks without pooping and then deposited a HOOOGE poopy. Then it was another couple weeks and then she pooped twice in one week. They go when they're ready to go. =)
It can vary a lot from snake to snake as they age as well.
When I first got Issa (my first ball python) as a baby, he would go about once a week.
Now that he's an adult, he's save it up for about a month and then let it all go in one huge mess.
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Depending on the size and age of your BP it could also be ready for something bigger than a fuzzy rat which is younger than a rat pup (which is about equivalent to an adult mouse). How does the rat fuzzy compare to the biggest girth of your snake sizewise?
Snakes don't go all that often really. As long as it's eating an appropriate prey once per week and has access to clean water to drink then it's likely going to defecate when it feels the need to. Does it pass urates okay?