Residential tenancies are typically 12 months long but can be up to three years. A tenancy in England or Wales longer than three years must be a deed, not an Assured Shorthold Tenancy (AST).
The longer a tenancy agreement, the more security landlords and tenants have. Tenants have peace of mind knowing they will be in a property they like for a long time, and landlords have the reassurance of a steady income and reliable tenant.