HI
Do you mean as heard or as displayed in the notation screens.
The note length of a midi note is the difference between when you first press the key (note on) and when you release the key (Note off). This is what you will see in the notation and these times a written in the file as data on the timeline.
What you hear will depend to some extent how much reverb you have on it and also the instruments note decay time.

Also where as a Piano note will die eventually even with sustain pedal down no mater how long you press the key down, a continuous voice like a flute will sound until you release the key.
Mike

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