The only thing that determines the leveling up, is the experience gain. Quest completion gives experience, and sometimes when you're close to reaching the next level, completing a quest might level you up... but there's no such thing as a "leveling trigger", and there's no specific milestones, where the game grants levels - every single level is earned with monster killing and questing. Just watch your experience bar, and also keep an eye on your quest journal, because the rewards of each quest are listed there, including the amount of experience you get for completing them.
