Since every radon atom decays into an atom of Polonium-218, a liter container of air with radon would soon have a large number of Polonium-218 atoms. However, because Polonium-218 is radioactive and has a short half-life of approximately three minutes, it will decay quickly after it is produced. Soon there will be as many Polonium-218 atoms decaying as there are being produced and the concentration of Polonium-218 will not increase further. This is called the state of Secular Equilibrium.
Radon is at secular equilibrium with its decay products when the radioactive activity of radon and its decay products are the same. It takes approximately four hours for this to occur in a controlled laboratory environment, such as a liter container where the decay products are not lost to plate-out or ventilation. For example, with a constant level of 100 pCi/L of radon, RDPs will achieve a maximum concentration of 1 working level in three to four hours so long as nothing is removing the RDPs. If ventilation or plate-out is removing RDPs from the air, there will be less than 1 WL for each 100 pCi/L of radon in the air.