The mean free path is the average distance a molecule will travel before colliding with another molecule. From kinetic gas theory (which is explained in any pchem text and probably some general chem texts as well if you're curious), we can derive the following equation for mean free path, λ:
λ = kT/√2σp
where k is the Boltzmann constant, T is temperature, p is pressure, and σ is the collisional cross section of the gas molecule in question. A good approximation of the collisional cross section is the actual diameter of a sphere representing the molecule in question. (In other words, you can estimate this with some idea of molecular radius and the formula for area of a circle.)
So use those powers of estimation to determine the mean free path for a Na+ ion coming out of the ICP (atmospheric pressure) and near the entrance to the MS of an ICP-MS (assume 10-4 Torr). Use standard temperature: 298K.