Well I'm not 50 yet on anything! But I've split my time between the DOM, DOH and BOT classes and occassionally been locked out of my main for days so...

I think a DOW/DOM is probably the quickest to level through Fate Grinding. I'd say probably 50-60 hours on your first class but the second and others will be 1/2 that due to the buff but there will be no more quests.

DoH is probably the next quickest but you'll need money to buy the mats you need off the market.

DOL is probably going to take the longest given the sheer amounts you'll have to gather.