Diffusion Magnetic Resonance Imaging (dMRI) encodes water displacement due to diffusion and is a powerful tool to obtain information on the tissue micro-structure. An important quantity measured in dMRI in each voxel is the Apparent Diffusion Coefficient ($ADC$) and it is well-established from imaging experiments that, in the brain, {\it in-vivo}, the $ADC$ is dependent on the measured diffusion time. To aid in the understanding and interpretation of the $ADC$, using homogenization techniques, we derived a new asymptotic model for the dMRI signal from the Bloch-Torrey equation governing the water proton magnetization under the influence of diffusion-encoding magnetic gradient pulses. Our new model was obtained using a particular choice of scaling for the time, the biological cell membrane permeability, the diffusion-encoding magnetic field gradient strength, and a periodicity length of the cellular geometry. The $ADC$ of the resulting model is dependent on the diffusion time. We numerically validated this model for a wide range of diffusion times for two dimensional geometrical configurations.