In a tracker, note lengths are typically not expressed like in sheet music but rather by having a note-on and note-off event. The note-on event determines the pitch of the note (e.g. C-5 or D#4), and note-off events (e.g. === or ^^^ depending on the tracker) stop that note in one way or another (e.g. cutting the sample off or slowly fading it out). The exact way to do that will be detailed in the help of the tracker you use.