Took me less than a week to level my MNK to 50... I used the hunting log to rush 15 then ran dungeons. The hunting log supplemented my XP as I went and it helped push me along.

The log was never meant to give a lot of XP... was meant as a small boost. Sometimes it is easier than hoping a level-appropriate fate is up in your area. And after the Atma grind pre-buff... I'm bloody sick of fates, so it serves as an alternative.