I agree 100% w/ Jade and Nancy. There is no way we can answer whether or not the plastic will pass on its own. That's why I asked you what kind of plastic your cat ate, to figure out how likely it will pass. I hope to God that it does pass. If it starts giving your cat problems, you will notice signs of an intestinal blockage-like loss of appetite, vomiting, and constipation. But, to ease your mind, why not just take the cat to the vet for an exam and some x rays. That's probably what I would do.
If your cat hasn't passed the bag on it's own by now, I would take he/she to the vet ASAP for an xray. You don't want to chance a blockage.
There is no one answer. As I said before, it depends on how much he ate and where it is lodged.