Um You've got a level 40 SCH right?

Doesn't that mean you leveled from 16-30 in the DPS queue as an Arcanist? Until you picked up the SCH job you were in the DPS queue.

In answer to your question an example of my experience as a BLM. I've been using DF to farm Brayflox (Hard) to get Soldery, and Mythology. Assuming I get on after work and go from 6-10 PM in game time and I queue back up for the dungeon after I finish one. I consider it lucky if it pops 3 times.

In the downtime while waiting for it to pop, I've gotten my arcanist up to level 29 or 30. So only 4 more levels, and then I get to switch and level up archer to 34. Fun fun.