The optimal plasma parameters for plasma processing, such as deposition of microcrystalline silicon from silane and hydrogen, are generally chosen in steady-state discharge conditions. However, this steady state must be reached in a short time after plasma ignition to avoid significant film deposition in non-optimal conditions during the plasma transient phase. Simple and inexpensive time-resolved optical emission spectroscopy has been used to measure the plasma time evolution from ignition to steady-state conditions in a large area RF capacitive plasma reactor. Absolute values of silane and hydrogen molecular number densities, relative values of electron density, and qualitative information on electron temperature were obtained without the need for absolute intensity calibration. Apart from the experimental verification of constant electron temperature, the particular condition here is that the emission intensities should be followed from the instant of ignition, since the molecular densities are known at this instant. A plasma model for the reactor, and a dispersive axial flow model for the pumping line, were used to show why the plasma chemistry in a well-designed large area reactor generally reaches steady-state conditions in less than one second. The optimal design for fast equilibration is a closed, directly-pumped showerhead reactor with a uniform plasma which fills the whole reactor volume.