A few sessions give you an effect that fades. A few months of it starts to change your baseline. Plan for the longer version if you can.

What the research says

In pooled randomised trials of exercise for depression, the effect held up after correcting for publication bias and was larger in supervised, higher intensity programmes. Those trials compared how hard and how supervised the exercise was rather than how many weeks it ran. So the general point that a more substantial programme yields more is supported, while the specific claim about duration is not directly tested.

Strength of evidence: one line of research. A single study, or a theoretical account.