Was wondering when is the best time to level a alt job?

I've been playing dragoon, but also want to try black mage or dark knight. Ive read you get a xp boost for alt jobs but I don't how or when you receive that perk.

thanks in advance

Any job that is a lower level than the highest level job you have leveled will receive an 50% exp boot. So level an alt job whenever you feel like it.

You always have that perk, whether you start your next job when your first is level 10 or level 80. All combat classes that are of a lower level than your current highest one get 100% XP boost up to level 70, then 50% from 70 to 80.

Niwashi's answer is a accurate. The XP boost is know as the armory bonus. It also stacks with rested xp, FC bufffs, and food buffs.

Nice Was gonna ask this myself.