I'm level 23 and I've only played like, twice. The sense I was getting was that I could basically level up as much as I wanted in however long I wanted to sit there in front of the computer and keep playing - but a big chunk of that was how fast the movement and dungeons reset, which I assumed (maybe incorrectly?) was set at an artificially-fast speed just for beta testing purposes, sorta the same way all phoenix feather stuff costs 1 feather right now. If dungeons/movement only reset once or twice a day it would slow things down.
Anyways, yeah, campaigns and sidequests give a lot of xp (I vote for crappier rewards for easy sidequests, and some harder sidequests [more like crazy achievements] that give decent rewards). I also feel like level progression doesn't slow down as steeply as it did in Estiah 1; the xp to next level increases each level, but not by a really large amount (maybe it gets crazier later on?) and so it stays pretty low even at the level I'm at. I also sort of feel like level doesn't matter much, at least so far - I'm mostly limited by my charm collection and my gold, level's really secondary. If you'd asked me to guess what the level cap was, just based on my feelings of how fast leveling has been and how "significant" each level feels, I would've guessed the level cap was intended to be a lot higher than Estiah 1's.