Anecdotally: I'm not the fastest or the grindiest person, but it took me a few weeks for each expansion step (15-50, 50-60, 60-70) to get all my crafters up. I used mostly leves, but a fair amount of collectables when I ran out of leves. So aiming for ~5 weeks of pretty hefty leveling would be reasonable.

Leve allowance rate is your biggest timegate here. Looking at this spreadsheet (use the tabs on the bottom to choose the job) it takes about 130-140 leves to get from 15 to 60, and this guide says another 48 for 60-70 using the most exp efficient leves. Sooo total is about 188 leves/class, subtract about 30 for starting at level 30, and

150 leves/class * 5 classes + 48 leves/class * 3 classes * 6 leves/day= a long fucking time, apparently (150 days, wew). Though you can probably remove a couple weeks from there if you already have 100 leves stockpiled.

But that's if you only used leves -

Ixal and moogle beast quests will make 30-50 and 50-60 considerably faster respectively

GC turn-ins reward massive exp each day

Collectables are available from 54-onwards if you have no leves left

And lastly there is passive exp gain, which you can boost with an item from your GC (or a better one purchasable with red scrips)