Jim,
well, I think it should be a user "choice" how long the preview is. I mean, we know how long it takes to generate 4 bars for preview, so I am guessing that "selected" 12 bars would take 3 times longer. The reason, (if it ever comes to life) to have preview "selection" is that I can see beneficial previewing 4 bars plus transition / break. And I think selecting a region is more user friendly than remembering numbers.