I leveled my first class from 1-50 in about a month.

I started as CNJ, spent about 7-8 hours a day playing relatively casually, making sure to do my daily roulette. I also quested, a lot, and took the time to level my crafting classes a bit, mainly because I fell in love with crafting from day one I will say, though, that I did have a bit of help from the refer a friend circlet and fc xp bonuses, though, so its not a solo level experience.

The main scenario, however, I didn't finish until like month 3. I got to lvl 50 before even beating Titan. Unless you care about battle class endgame, the main scenario is relatively unneccesary.

Also, leveling multiple classes slowly is REALLY inefficient. higher level cross class skills are amazing, and you can help your low levels much easier with a single level 50 character on-hand.