The police have until the applicable statute of limitations expires to charge you with a crime, presuming they can show probable cause. While I don't know the precise statute of limitations for the crimes you mention in Pennsylvania, I can tell you that statutes of limitations in all states are measured in years, not days or weeks. The officer clearly wanted to give you a chance to reconsider your decision not to work as an informant for them. He was within his rights to wait to file charges. Some states, however, have statutes requiring officers to cite you immediately, on-scene, for motor vehicle charges. If you were not cited at all, then you may have defenses to those charges. Speak to a local criminal defense attorney.
No. THe statute of limitations in this case is 2 years. No, this is not entrapment. Entrapment occurs when the government takes action which would cause an otherwise innocent person to commit a criminal act. Here, the police did not cause you to possess marijuana or paraphernalia. Instead, they offered you the opportunity to be a "snitch." You chose not to do so.
Nothing in this factual scenario would cause the charges to be dropped. However, this question seems familiar to the series of questions that was asked last month. Although I certainly understand your concerns regarding the cost of a criminal defense attorney, the answers on this board are not a substitute for the advice of an attorney who is familiar with your case, as well as the jurisdiction, and venue of your case.
Call a number of attorneys and make the best possible choice. Criminal defense is not a do-it-yourself project. The stakes are too high.
This answer is for informational purposes only and does not imply an attorney-client relationship. You should consult with a local attorney.