Final answer:
Registrants are required to conduct an inventory of all controlled substances every two years.
Step-by-step explanation:
According to 21 CFR, Section 1304.11, Inventory Requirements, registrants are required to conduct an inventory of all controlled substances no less frequently than every two years.
The inventory requirements for controlled substances vary by country and are typically governed by national or regional drug enforcement agencies. In the United States, for example, the Drug Enforcement Administration (DEA) provides regulations and guidelines for the handling of controlled substances. It's important to note that regulations may change, so it's advisable to consult the latest guidelines from the relevant regulatory authority.