We present useful expressions predicting the filling time of gaseous species inside photonic crystal fibers. Based on the theory of diffusion, this gas-filling model can be applied to any given fiber geometry or length by calculating diffusion coefficients. This was experimentally validated by monitoring the filling process of acetylene gas in several fiber samples of various geometries and lengths. The measured filling times agree well, within ±15%, with the predicted values for all fiber samples. In addition, the pressure dependence of the diffusion coefficient was experimentally verified by filling a given fiber sample with acetylene gas at various pressures. Finally, optimized conditions for gas–light interaction are determined by considering the gas flow dynamics in the design of microstructured fibers for gas detection and all-fiber gas cell applications.