FFXI is more of a long-term investment type of MMO. If you suddenly have a small bit of free time there are other MMOs that might be more satisfying. FFXI is designed for long-term commitment and hardcore players. There is much less to get out of it in the short haul. By the time your life gets busy again you'll either:

a) be too addicted and let it interfere
b) not have made enough progress to get to the meat of the game

I'd say that at least a year is reasonable for most people to get to 75. You mention 30 as being a spot where you really feel like you're part of it, but it's an old game without a huge influx of new players (compared to other MMOs). Most people are at the higher levels and content is geared totally toward them.

You can't get deep into nation story lines until your 60s at least. You can't finish Zilart until your 70s unless you get lots of 75 help and do it sooner. You can't finish CoP until you're 75... not even leeching off of people. You can get more out of ToAU more easily, but in the end you'll still need the levels to do the fights.

If you don't plan to play long term (2 or so years) or if you have plans at some point to ever have a busier schedule, you may want to just pass on this particular game. It's a great game and SE is making it better constantly (after years of apathy), but it's not at all for the casual gamer. I know some people enjoy playing it casually, but they are in the vast vast minority. I wouldn't expect most people to be able to handle such slow progression coupled with the elitism of such a hardcore game.