when to start a new class

[Anova]

Generally, DPS have a little more leeway when changing to Job. Most will start leveling a second class so they can get a job by level 35. The job gives you new damage mechanics you absolutely want to use by level 35 or 40.

For Monk, the ability to quickly retreat, initiate, and switch targets at a distance is a huge one if you're trying to keep up greased lightning after lv.35. The lv.30 ability, Rockbreaker, is sometimes used in very specific speedrun and dungeon tactics. Otherwise, it is largely ignored.

The other matter of contention are attributes. It's not a huge difference below level 40. Tanks and Healers should job change as quickly as possible simply because tank/healer job abilities and attributes are useful starting from lv.30. Most players still use their classes to level when solo until lv.35 though, especially if they have a lot of cross-class skills.

Once you reach level 50 though, you will want to start leveling your cross-classes to 30 or higher. This is mostly for secondary abilities that augment your damage, improve tankiness, or improve your support abilities. There's a number of difficulty spikes after 50 and those abilities are recommended for you to perform well and succeed.