A character reaches 1 checkpoint for each hour an adventure is designed to last. If a character completes an adventure designed for a higher tier than the character's current tier, the character is awarded one additional checkpoint (may not apply to AL if tiering remains).
At levels 1-4 it takes 4 checkpoints to advance to the next level.
At level 5 and above, it takes 8 checkpoints to reach the next level.
Basically you level up every 4 hours in tier 1 and 8 hours thereafter. This is true regardless of whether or not the characters accomplish anything or just hang out in an inn.