There really is no "M level" equivalent, since the blinds don't rotate. But you should know when your stack is reaching various fold
equity points.
1) When you don't have enough chips left to put in a bet or raise on the river (or when your stack hits 4-5 big bets). Depends on position.
2) When your stack isn't deep enough to put in a full bet on 5th or raise on 5th. Depending on position, 2.5-3.5 big bets.
3) When you only have ~1.25 big bets left, so you can make iso-raises on 3rd.
These are the main points in a hand when you have fold equity, or when
hands change in value (draws/speculative hands go down in value if you can't bet the river when you hit). But the stud games are limit games, so your fold equity doesn't quite fluctuate as much with stack size. The important thing is just to play 3rd street well.