Final answer:
A broker-dealer is permitted to hold a customer's mail without instructions for a valid reason for a period of 90 days.
Step-by-step explanation:
A broker-dealer is permitted to hold a customer's mail without instructions for a valid reason for a period of 90 days. This is in accordance with the rules set forth by the Financial Industry Regulatory Authority (FINRA), specifically Rule 3150. This rule allows broker-dealers to hold mail if there is a valid reason, such as the customer being unavailable or unresponsive.