Final Cut Pro X Custom Length Hold Segment
I use freeze frames all of the time. You can make one by placing your playhead or skimmer on your chosen frame and from the Edit menu choose "Add Freeze Frame". Or use the keyboard shortcut Option - F.
The duration of your Freeze Frame is determined by the "Still images: Editing duration is" setting in Final Cut Pro X's preferences.
You can also make a hold segment instead of a freeze frame. Unlike a freeze frame, a hold segment is live. Any change you make to its parent clip, like color correcting, will be telegraphed to the hold segment.
Place your playhead or skimmer on the frame that you want to hold and from the retime dropdown menu choose Hold. Or use the keyboard shortcut Shift - H.
The default hold segment is 2-seconds. You can change this length by dragging the edge of the hold segment in the timeline, but you can not change the hold segment default length of 2 seconds.
If you'd like to make a custom length hold segment, select a range in the timeline...
...and choose Hold from the Retime dropdown menu. Or use the keyboard shortcut Shift - H.
Your custom length hold segment will be the length of your range selection. The first frame of your range will be the hold frame.