The paper presents two models that allow us to determine the change in the brightness of a comet during its outburst. The first of them (model A) is associated with the impact of individual scattering cross-sections on the brightness jump of the comet. The second (model B) is associated with the increase of sublimation active surface during an outburst. Based on these two models, we determine the change in the brightness of comets during the outburst. We assume that cometary particles consist of ice, dust, or their interconnections. For water–ice particles, the key factor is determining the lifetime in a coma. We find that the cometary outburst phase lasts until a significant amount of water–ice in the coma runs out. The loss of water–ice due to sublimation reduces the surface on which the incident sunlight is scattered. Consequently, this causes a slow decrease in brightness to the level before the outburst.