In my case, I have always known exactly when my passport expired.
My cruise vacation starts June 9th.
My passport expires 3 months after the cruise so I figured I was good to go.
Then when I learned I needed 6 months left on it to travel, (as told by Celebrity), I immediately filed and sent my current passport in and paid for expedited and 2 day mail.
I only began to worry when I read about the glut of applications and the increasingly longer wait times with each passing month.