The knowledge of the time to reach a new equilibrium after a hydraulic perturbation is crucial to model accurately the groundwater system. A hypothetical numerical model loosely based on the western margin of the Great Artesian Basin in Australia was used as a demonstration to estimate this time in large unconfined/confined aquifers. The time to reach a new steady state after a hydraulic perturbation, such as the cessation of the recharge in arid and semi-arid regions, is on the order of 50 ky. This time is longer than the present interglacial phase, i.e. 10ky since the last climate transition, and thus the hydrodynamic system is a transient one which is still responding to the paleo-perturbation due to recharge. Moreover, analytical solutions have been used to assess the time to reach a new equilibrium for several large aquifers in the world. The times obtained range between 0.4 and 5.8x1010 years. These values suggest that large regional scale aquifer systems are rarely expected to be in steady state with respect to their hydraulic behaviour.