Starting 11: No less than 91% - Injuries and Suspensions will alter this.
To be a sub: No less than 85% - I rarely need to go this low, though.
To be subbed: If the player gets to 69% or below I'll sub them, depending on other players' fitness.
All of this depends on the players' stamina. Player with high stamina and 91% fitness will always start if they're good players. But, if a player has lower stamina with 91% fitness I'll put them on the bench, as they'll have to come of around 60 minutes.