We know a lot of information about FFXIV but, sadly, all of the most important information -- chiefly the amount of engaging content -- we have no idea about.
It would be safe to say that leveling will be a pretty standard experience when compared to most post-WoW MMORPGs. Simple quests that provide a wall of text, a rudimentary task, a chunk of experience, and one of two selectable rewards will be plentiful and standard (whether from NPCs or as guildleves). We know that there will be fairly typical instanced raids, so you can make of that what you will. But we also know that there will be a chocobo-raising system and a housing system, which may provide a reasonable amount of side hobbies.
Approaching the question as objectively as I can, I would say that, in FFXIV, you will probably have to work harder to achieve a "top-tier" character than in WoW, if for no other reason than the fact that you'll need other classes leveled for cherry-picking useful skills. In 1.0, seasoned players could take a few months (it varied widely between, maybe, 2 - 6 months) and really do just about everything important in the game. I never reached 50 in multiple classes, so take what I say with a grain of rock salt (my only noteworthy levels were a Pugilist ~40, a Thaumaturge ~30, and an Alchemist ~25, which in my opinion was more than enough). In that there are many classes in FFXIV, there is a lot of "content," but I think we need to distinguish between "content as engaging or varied activities that make you want to keep playing," and "content as repetitive stuff that, while not particularly appealing or variegated, can be done."
Ultimately we don't know the longevity of FFXIV because it's not officially "reborn." We can logically infer, though, not only from developer descriptions and videos, but from 1.0, that the game will feature many casual-friendly elements. As of this moment, there is little reason to think that FFXIV will be (re)released with a year's worth of challenging content waiting to be explored. However, it will quite likely necessitate somewhat more investment than the ~1-2 months that many fantasy-themed, genre-standard MMORPGs require of their dedicated players.
____________________________"... he called to himself a wizard, named Gallery, hoping by this means to escape the paying of the fifteen hundred crowns..." (Machen 15) "Thus opium is pleasing... on account of the agreeable delirium it produces." (Burke para.6) "I could only read so much for this paper and the syphilis poem had to go."