Final answer:
The NFPA data required to determine the frequency of structure fires in the United States is not supplied in the question or the references provided. To answer the question, current NFPA statistics are necessary.
Step-by-step explanation:
The frequency of structure fires in the United States, as per the National Fire Protection Association (NFPA) data, is not provided in the question or the reference information given. While the mentions of a serial arsonist in Hollywood, CA, and the rise in human-ignited fires affecting forest cover point towards discussions on fire incidents and their consequences, they do not offer a specific interval for how often structure fires occur. To answer the student's question accurately, additional current statistics from the NFPA would need to be consulted.