I sub any player that receives a yellow card immediatley.
If i'm not winning, and i don't see the name of a player in the comments, and he usualy is there, i sub him.
And i also sub, if i have to change the formation, and don't have the right player on the field.
I always leave a sub untill the 81 minute, and then i make the 3rd sub, usualy subbing the good players, so no troll injuries happen in the 89th minute.