Important rule:
If the part of your game is not fun nor challenging and does not utilize game mechanics potential you should make it last short (generally you should remove it but MMORPG designers are going to say: it was in WoW so if you want to call your game MMORPG you have to make it feel like WoW).
If the part of your game is fun, challenging and does utilize game mechanics potential you should make it last long.
So why the hell IMC thinks that making leveling part take huge amount of playtime is a good idea?
Besides, quest grind is a bad idea overall. I cannot think of a single game where it was working good. It is ruining leveling experience, lore experience, interaction with the game world, everything. Just a cheap move to pretend that leveling is not brainless annihilation of mobs i suppose.
