Final answer:
The normal length of time for developing a marketing plan is typically one year. This period balances responsiveness with long-term strategy and encourages research and development.
Step-by-step explanation:
The normal length of time for which a marketing plan is developed is typically c. one year. Marketing plans are strategic documents that outline a company's marketing efforts for a specified period of time. A one-year plan is common because it allows for a balance between being responsive to market changes and setting stable, long-term marketing strategies. It's beneficial as it promotes research and development by the company of its product, which can lead to improvements and innovations that benefit many customers.