Board designers often ask this question when they’re looking for a starting point for their CAD drawings or simulations. Because there are so many variables to consider, it is difficult to provide a "correct" answer. Clock speed, 1T or 2T timing, registered or unbuffered modules, and trace impedance are all important factors. Some controllers have on-die termination, some do not. Some controllers have two copies of the command and address bus. All of these factors can affect trace lengths and termination and can affect how acceptable signal integrity is achieved.
Micron technical notes TN-47-01, TN-47-20, and TN-46-14 can be used as a starting point, but trace length and termination must ultimately be proven by simulation and physical testing. Micron provides an online simulator for customers who do not have the expertise or resources to run simulations. The online simulator is on a secure section of Micron.com; visit the following URL to request access: www.micron.com/simulators.