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