A Foresight exercise typically lasts between 6 months and 3 years. In some cases it can become a continuous activity.
Under certain circumstances, where the subject is quite narrow, Foresight projects have been completed within two weeks, based on two half-day meetings, especially in corporate Foresight exercises.
When deciding on the duration it is crucial to be realistic. There is a natural tendency to overestimate the amount of work that can be done within a given time span and to underestimate the difficulty of creating or moving things when various stakeholders with conflicting interests are involved.
Later on, when running the exercise, managing time is one of the major challenges.
There is more information on deciding the time frame in some of the example cases: