It's not exactly obvious... But I have found the source tree of touchscreen-keypad lock screen. Though, I still don't see when and why it decides it is no longer needed.
Without lockscreen. User active: screen bright. Dim_timeout seconds without user actions: screen dims. Blank_timeout seconds of dim screen without user actions: screen blanks.
With lockscreen. TKLock screen waits for about five seconds for user to start acting; then the screen, without changing its brightness, blanks. I have not found where the value for its timeout is written.
Best wishes.