It depends on how you play the game. I find it utterly pointless to keep changing jobs whenever I master one job. I map out what I want my characters to do, and take the necessary steps so that they become what I want. For example, take what I did with Ramza:
I wanted a fighter who could equip Two-Swords, so Ninja. I didn't want any other Ninja skills, so I only learned the Two-Swords ability before abandoning the job.
I decided that I wanted Ramza to specialize in his class, i.e. Squire. Therefore I mastered squire (even though that was probably unnecessary, just so long as I had Gain JP up, Move +1, and all the abilities). I also wanted Ramza to have good firepower, so I chose to master Oracle, Time Mage, Wizard and Priest, then Calculator.
Finally, I spent some time with the Bard class for Move+3, and I aquired Auto-Potion with the Chemist class. It took a long time to build up because of all the Calculator preparation, but it was worth it in the end.