Though nonsolvent-induced phase separation has been used for decades to prepare polymeric membranes, it still remains a challenge to have clear insight into how the porous structure forms during membrane preparation. The presentation focuses on a point that is usually overlooked in the literature: phase separation may need time to occur. We found that the times, given and needed for phase separation to occur, play important roles in the formation of membrane pores, and they are strongly related to the degree of polymer chain entanglement in the casting solution, which can be well characterized by solution rheology. Examples will also be given to show how the knowledge about the phase separation time scales led to preparation of porous membranes with high inter-pore connectivity and polymer membranes with super-hydrophobic properties.