Hi, SJCryptic,
The absence of a duration label on a clip in the timeline has been reported by several posters here following the update to iMovie 10.1.15. It appears to be a glitch that Apple will need to fix.
Some posters have found that by placing the cursor on, or clicking on, the very last frame of the clip will reveal a pop-up of the duration. I am not running iMovie 10.1.15, so cannot verify that. But you can try it and see.
Another workaround is to select the clip and click on the information button (the circle with an "i" in it) in the toolbar at the top right hand corner of your screen. That will reveal a duration box just below and to the right of the toolbar, that displays the duration of the clip.
Still another option is to use a Time Machine backup, if you have one, to restore your iMovie 10.1.15 to the previous version that you were using.
You might also try deleting preferences, as often that works to cure issues. Open iMovie while holding down the Option and Command keys and select to delete preferences from the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the duration label appears on the clip.
-- Rich