It's SetDaySpeed but the variable isn't in the pc .ini as standard. The setting is for the number of seconds it takes for an in-game day to complete.
The format is:
SetDaySpeed(number of seconds)
This is to be placed below the MaxThingDrawDist line.
I've not tried it on the pc but my old xbox ini looked like this:
SetMaxAnisotropy(4);
RunScript("joystick.ini");
SetMaxAnimatedMeshDist(64);
SetMaxStaticMeshDist(128);
MaxThingDrawDist 128;
SetTimeOfDay(7);
SetDaySpeed(86400);
ActivateQuest("Gameflow");
The number of seconds above is the equivalent of 24 hours.
Feel free to give it a try.