How can that be answered? All the advanced jobs are different, many depend on luck or what your old job was (for instance, being a white or red mage lets you go anywhere easily). The question cannot be answered as hours or days. Ninja, for instance, takes factioning. Summoner takes the random chance of finding a rare drop - I got mine in ten minutes solo, I was lucky, others have tried for days and gotten nothing.
There isn't a job out there that can't be finished in three hours (if not less) given the right kind of preparation, help and luck. (for instance, getting the fame or getting the ruby before pursuing the job).
So: three hours. more if things don't go perfectly.
Beastmaster is the easiest advanced job, just takes running from Jeuno to Qufim and back. Took me about ten minutes start to end.