On the Subtitles dialog, the items above referring to 'In' and 'Out' point fields, will appear as, and edit, the 'Show' and 'Hide' point properties of the current subtitle.
In Storyboard mode, when entering a gap duration, each gap can be a maximum of 5 hours in length.
Note that most users will never need to create or modify gaps longer than a few seconds each.
This method of editing clip lengths is best when precise lengths have been predetermined. If you prefer to line your in and out points up to visual cues on the clip, use the clip navigation buttons in the Selected Clip Preview area to cue up the precise moment of the in or out point, then use the red and blue flags to mark the in or out point.