How often do you use milestones for levels ?

I ran a very short campaign of a couple sessions and tried the milestones system. I quite literally had players find round stones writen "mile" on it after solving puzzles or inside the corpse of hard enemies (which led them to dismantle every creature they killed no matter the size). I think the system worked quite well but my milestones were always giving a full level so needed to be scarcer and scarcer giving less progression feelings to the players.

What's your experience with it ?