Disclaimer: This is all based on my opinion, i am only sharing these thoughts to help those of you having a hard time grasping the difference in the situation regaurding populations and the war effort.
Ok, so the way i see it, Each Expansion pack takes place over a certain amount of years.
Origional/Vanilla WoW: The Adventure explores azeroth, starting the grandest adventure of the century in an age of hero's. this phase of World of Warcraft takes place over the course of maybe 4-5 years, and our adventurer starts at a young age ( 18-20 and up ( obviously, older if your an elf or something similar) )
The Burning Crusade: We have been called to war by our respective nations, the alliance and horde march together on outlands, but have conflicts along way. after many harfought battles with and against your opposing factions, you finally breach the black temple and defeat illidain. your next challenge is the Sunwell, where you face the powerfull demon achimond. I'd have to say this phase takes at least 6 years.
Wrath of the Lich King: The war between the alliance and the horde has gotten more serious. to make matters worse, the Lich king has attacked the major city of both the alliance and the horde. The Campaign on Northrend begins. Both factions make new allies and march across Northrend, skirmishing with eachother and the scourge. Naxrammas, Malygos, The Black Dragonflight, Even the god of death, all are defeated by your faction. the time finally comes to assult Icecrown Citadel. The alliance and the horde cause trouble for eachother and, at the same time, push further into the citadel, untill finally, the lich king is reached and beaten, but not destroyed, for there must allway be a lich king..not long after a new threat arises, Hallion, a Twilight dragon of immense power, a herals of a new age, a Herald of Cataclysm. this phase probably takes another 8 years.
so i would say that between vanilla and Cataclysm, about 20 years has passed.
as a side note, does anyone know how many hours compirses a day on the game? like how long does it take the game world to shift from day to night and back.