Checking duration of a marker-created subclip in Final Cut?
The reason you cant get the durations is because a marker is just reference to a point in time, not a time range. This time around you will need to find the durations manually either by opening the clip or subtracting the time of the previous marker form the current one. 

You can give markers durations, but not as effortlessly as just placing a single marker while watching through the video:


  Place a marker by pressing M then at the end time hit
  Option+M, this will extend the duration of the
  marker to wherever the playhead is.