Under the historic common law, 99 year lease referred to the longest term of a lease of real property. Even though it is not the law, in some jurisdictions the practice continues. However in most cases, the 99-year term appears to be more of a custom than a legal requirement.
In the U.S. most states have no limit on the length of the lease. The parties are free to contract for whatever term they desire. A few states have enacted statutes limiting the length of leases to 99 years. For example, Alabama limits all leases to a maximum of 99 years.
Example of a State Statute _ Alabama_ on 99 year lease.
Code of Ala. § 35-4-6 Leasehold estate for years; duration and voidability.
No leasehold estate can be created for a longer term than 99 years. Leases for more than 20 years shall be void for the excess over said period unless the lease or a memorandum thereof is acknowledged or approved as required by law in conveyances of real estate and recorded within one year after execution in the office of the judge of probate in the county in which the property leased is situated.