Influenza viruses can escape from antibody responses over time owing to their high mutation rates. Typically the flu jab offers protection for between 6-12 months. As the vaccine protects against different strains, and the protection against some of the strains may last longer than others. Most protection wanes within one year. Currently, annual flu vaccines are required for sustained protection against flu.
Longer lasting flu vaccines, that may not need to be updated every year, are under active investigation. Progress in vaccine development during the coronavirus pandemic may speed their development but we are still at a research stage and so it may be some time before these are available to the general public. It will be a great advancement if achieved.