How many minutes must a player play to prevent unhappiness?

I have a relatively big squad and some of my players complain about not getting first team football. Now I found out that their squad role seems to play a big role in their demands. So "hot prospects" usually never complain. But how many minutes of playing time does it need to satisfy backup, rotation, first team and key players? Or do they need to play a certain percentage of minutes? Also, do hidden attributes (ambition etc.) play a role in this?

yes, hidden attributes play a role. so the answer depends on those.