This question has no easy answer and yet it does:
- as much as you want
The main variables and sub-games might be ignored or properly managed. They include:
- market shares, monopolies
- province improvement (much simplified vs city managment in civilization)
- societal slider gamer (longe duree)
- military builtup for land and sea
- warfare, land and sea
- colonialism, trad-based
- colonialism population based
- diplomacy
- stability
- religion
all this while managing your inflation and budget.
The budget ties that all together, for obvious reasons. So at first you can concentrate on the budget management and see how you can increase it to actually accomplish stuff.
It greatly depends on the size of the nation you play, too!
My greatest fun is to play Byzantium and to turn the tides of history. Konstantions XXXXI, victor of the Battle of Austerlitz and emperor of all of India!
You get the idea.
If you wanna accomplish a lot or play long shot games, the optimization of all subsystems need proper investment of time and thought.
But at first you should try to overcome the main frustration barrier: keeping a country from debt and inflation generating a small surplus. Try to manage that, and it will lead to better understanding of the whole, gradually increasing your mastery and ultimately the power to pursue the goals you set yourselves.
I find the task of managing debt and inflation so insight/interest-generating I will say EUII is a worthwhile tool for personal and intellectual development.