Coring takes between 3-20 years in EUIV.
Coring is done actively rather than passively, costs something (besides reduced income and 30 years of nationalism), uncored provinces give scaling overextension penalties, exceeding 25 base tax of uncored provinces unlocks high frequency penalty events, and you no longer get free cores from missions and events.
Basically, no one would conquer anything directly if they had to wait 50 years or suffer massive penalties. In fact, in version 1.0 vassal feeding was pretty much essentially if you were going for a truly sprawling empire.