I've been playing for a little over two weeks now at what I'd consider a "normal" speed (couple of hours a night during the week, more at weekends) I've got two War classes at 40 so far (started as Pugilist switching to Rogue, got bored and swapped to Lancer) and I'm just trucking through the main quest at a reasonably casual pace. Considering the amount of basically unskippable main quest scenario stuff I've had to do to unlock everything, I don't believe for a second that a new character could blitz their way to 50 in three days without substantial assistance from people rushing them through all the dungeons and stuff. Hell, I didn't even get my chocobo until about level 25 and that in itself is a huge timesink when you're forced to run everywhere on foot.

Sure, if you've already done all the unlocks on one character and you're just spamming hunt log entries and FATEs to level another class, it'd probably be quite a lot faster, but definitely not for a new player.