The right roasting is how long? 8 minutes or 20 minutes?

there isn't one right number. Specialty coffee mostly roast in range 9-14 minutes for a medium batch, but that's not absolute rule. What's more important is what happens during Zroasting: when first crack, how much Zdevelopment time comes after.

eight minutes. That's usually fast roast, there's this school of thought that likes to keep this for preserve brightness. That 20 minutes might be slow roast or there extended drying phase. They both make good coffee when they know the purpose.

roasting under that 8-minute zone starts in because the heat comes in too fast and the development is uneven. roasting above 16-17 minutes for that specialty to be able to get into the baked territory. But among the range there are many valid variations.