Until when more or less do I expect my players to be fatigued if they play a ligue match at 18:45??

It depends on your player's endurance and your health center...

Also the duration they are on the field (and the match type, league in this case).

Is it duration on the field, or as a result of how many actions they take? I seem to see a large variance in the fatigue levels after a match, so assumed the more fatigued ones did more during the match. But I haven't really checked it closely, so I suppose it could be just because of their differing endurance levels.

To be exact, I suppose the duration reflects the distance a player has covered on the field. I think you have a point also with the actions, but I'm not sure about that.

This is only for U21 players: "If a player has a high match bonus, his endurance will be lowered resulting in a higher fatigue."

http://rockingsoccer.com/es/futbol/help/home/category-2/article-43

I have seen some of my youngsters, with high grey star values get very fatigued in some matches. So that explains some of the variance there a bit.

Would still be interested to know if moves on the field or actions taken have any bearing, or if it is just time on the ground.

I have noticed that players with Sprinter special ability get more tired (after gaining that skill)

/Roope