Marginal regeneration is the process at the origin of the thinner patches produced at the bottom of vertical soap films and slowly rising to the top. It controls the film drainage, and thus its lifetime, and ultimately the life span of the whole foam sample. Despite its importance, the dynamical properties of this original hydrodynamical instability remains to be elucidated. Using a deformable frame, we identify a way to trigger the instability in a horizontal film; as the gravity-induced rising motion of the patches is suppressed in this case, we are able to study its long time dynamics for the first time, in a well-controlled situation. The predictions made by Mysels et al. [Soap Films: Study of Their Thinning and a Bibliography (Pergamon, New York, 1959)] are successfully tested on our data and are extended by an original coarsening model we develop. This model quantitatively reproduces the patch growth in the horizontal geometry and, when adapted to the vertical situation, is consistent with the known properties of the instability.