OK, enough about this 90 in a day crap...I was just looking at the field manuals, and technically, it may well be possible to do this. Granted, one may need to be at it for a full 24 hours b/c of getting the brakes put on at the initial GK stages, but that is just a speed bump if you have help doing it. Otherwise, 2 or maybe 3 days to get from 1 to 90.
As for empy weapon, while it is more of a grind...the point of the original post is to show how absurdly easy it is to get a job leveled and geared. Doing all the additional content is a completely different issue, and not necessary to get a job to 90 and (except for some magian targets) not needed to gear it.
1,565,350 xp total to hit 90.
100,550 xp total to hit 30.
1,464,800 xp total to get from 30 to 90, which can be done easily as a leech.
One Saturday afternoon off with a PL friend and you can get from 1-30, and then if you get into a leech spot...well, except for stopping to run down the GK quests....welcome to 90. If you already have the limit breaks from another job....well, you could unlock a new job, level it to 30 with a PL, then overnight AFK it as a leech and wake up to a level 90 job.
30 in a day, impossible you say? Technically, it IS possible now that you get double xp/kill--remember, that calculates to 200 on an EM, plus you can get the chain bonus on top of that. If you were to manage to kill a T at level 1...welcome to level 2.
For the first 8 levels, you can hit average upwards of 500 xp per page from FOV right outside your starter city (270-330 bonus, plus xp from mobs). This isn't accounting for chain bonus either. After this, EM+ is no longer an option...you can stay there and grind it out to level 17, or change area and keep the potential for EM xp going.
Move one zone away and get your next 6 or 7 levels...FOV rewards go up now from 340 to 450 a page...so now your getting 750+ a page on average. Again, with a little help, this won't take too long to get your next 15/16k xp--also, this is not accounting for chain bonuses when you are hititng EM+. Once you run out of the EM potential here...either grind it out to hit 22/23 or move on to next area.
Now, you are getting to the good stuff... welcome to the dunes or buburimi peninsula. Dunes is nice since you have the subjob quest right there, and the book at OP is positioned nicely for spamming pages--everything you will need is essentially right at your finger tips. 475-570 Bonus xp per page, nearly all of which can be down within roughly 1 square of the book. The next ~40k or so here still goes by pretty quick at upwards of 1000 xp/page with no xp band...can easily average upwards of 1500 per page with a little help...up to and over 2k a page if you are still able to pull off EM+ fights (again, that's not including the bonuses if you are chaining them).
By now, you are level 24/25. If you've done it right... you have all your items to unlock subjobs. Whether you stop and do a subjob now or not could cause a detour. Once again, no more chance for EM+ chaining now, but you CAN opt to just grind it out to hit 30 right there (can actually take you to 35/36 technically)
If you want to stay with EM+ hunting...on to next zone. From 23/24 to 30 is roughly another 40k xp. Using one of the 3 zones right outside Jueno, you get 600-800 bonus xp per page, depending on which zone you use. Which is best is open to debate, as you will have a little running around involved. This may prompt more to just grind out the xp in the Dunes or deal with the roughness of Qufim's eventually, who knows. Anyway, if you are sticking to EM+ targets, it's 1100-2600 per page, depending on how you min/max it--that's with no xp band, and no chaining calculated in there.
No special keys needed, no special quests needed. At worst, you might have to get someone to run you some gear (maybe just borrow stuff from your friend who is helping you out--just trade it in the field and keep going). 1-30 is possible in one day with the right help. After that, one can the leech. Getting help with the initial GK's and the Maat fight (testy farming.. uggh) that will hold you up.