Any gas or vapor, by definition, is NOT held together due to its own inter-molecular forces of attraction, which is the case for liquids and solids. Therefore, it must always be stored in gas tight closed containers. If any leak exists, by the process of thermal diffusion, it will escape to outside, no matter what may be the differences of pressure or temperature between the inside and the outside.The size of leak and the pressure difference (including the influence of any difference in temperature and concentration) will decide the rate of the said (inter) diffusion. The time of say one third of the gas escaping from an open vessel may be a few seconds to weeks, depending upon operating parameters, e.g., the difference in densities, the orientation and the size of the opening, etc.Interestingly, even a lighter (than the surrounding air) density gas will diffuse out through any opening in the bottom of the vessel. Likewise, a higher density gas will diffuse out through any opening at the top of the vessel. The molecules of all gases are constantly and randomly moving with velocities of hundreds of meter per sec, even at ambient temperatures. That explains these peculiar properties of gases.