What type of leasehold estate has an unknown duration at the time of creation?

A leasehold estate with an unknown duration at the time of its creation is referred to as an estate at will. This type of estate is characterized by the agreement between the landlord and tenant, which allows the tenant to occupy the property without a fixed term. This arrangement can be terminated by either party at any time, making the duration uncertain.

The flexibility of an estate at will makes it distinct from other types of leasehold estates, which typically have defined time periods. For instance, an estate for years has a specific start and end date, while an estate at sufferance occurs when a tenant remains in possession after the lease has expired without the landlord's consent, but also has a specific contextual duration tied to the preceding lease. A nonfreehold estate is a broad category that encompasses various forms of leasehold interests, but it doesn't specify the unknown duration that defines an estate at will.

In summary, the key feature of an estate at will is its indefinite character, allowing tenancy to continue without a predetermined end date, thus providing both parties with flexibility in managing the rental agreement.