I'm having the exact same issue as well since updating to CC2018. I tried two things first that should have worked: (1) changing the units under preferences and (2) right clicking on the ruler and changing it to inches. No luck. Here is what worked, oddly: Start a new document (File>New), in the window that pops up, set the units to inches. Then click create. For some reason that works, it keeps those settings even after closing and reopening.
THIS WORKED FOR ME TOO:
I'm having the exact same issue as well since updating to CC2018. I tried two things first that should have worked: (1) changing the units under preferences and (2) right clicking on the ruler and changing it to inches. No luck. Here is what worked, oddly: Start a new document (File>New), in the window that pops up, set the units to inches. Then click create. For some reason that works, it keeps those settings even after closing and reopening.