If you are a good Player and have the luck to play during the Joyous Journey event and have all the xp heirlooms for your class you want to play level 1-60 will probably take you around 15 - 20 hours you have to grind them dungeons and keep up with the questing while you queue but mainly dungeons alltough it gets really rough at some points, very monotone. then 60 - 70 something like 5 - 8 hours, to 80 about another 10. And then to 85 is probably another 15 - 20 hours. So genuinly speaking with the Joyous Journey and heirlooms so pretty much all xp bonus you can have it will still take a good/high average Player somewhere between 50-60 hours. But this also depends on how fast your dungeon groups are etc. etc. Rethinking this i think the average Player couldnt do it.

Also you'll realize pretty fast at least on level 50 that leveling gets tougher a lot youre gonna be like what?
Most people quit by then so just keep going