Sorry, I think based on the answers I haven't been as clear as I could be. Regardless of XP vs Milestone, my question is more focused on the length it takes in game to level up.
For example, I'm trying to implement a rule, where when one levels up - they are to do it during a couple of days in game time. Rather than immediately have them move from level 2 to level 3 within a dungeon. My reasoning for this is that I think it's a bit unrealistic to immediately gain a new set of skills just halfway through the day.
It was the above that I wanted advice on, not out of game levelling.
My bad!