When entering a Neck Position point that is valid until the end of the song, there’s no need to manually enter an End Time for the point. Just leave it empty (--:--:--.---), then start and stop playback. The End Time for that point will automatically be set to the end of the audio file.
Note that you can have as many Neck Position points as you like in an Instrument channel. Usually, one is enough. However, you might choose to “focus” the view of the neck on different sections at different times, such as (for example) frets 1-5 when all the chords are played low on the neck, and frets 10-15 when a melody or solo is played higher on the neck.