Long of short is relative
Prophecies is by far the fastest campaign to complete if you skip things, I can finish it from pre-searing in a few hours (TH then 3 missions to end).
Actually playing the campaigns, and enjoying the storyline along with the myriad of side quests, takes considerable time if you explore to a reasonable extent.
Those of us who have finished all of them many times, can breeze through them all in a matter of hours by running or other short cuts.
What really matters here, is that you, as a new player will find the game world vast and varied, replete with more than enough to keep you busy for a long time if you play in the right spirit.
EoTN, apart from being a lot of fun in its own right, provided extra heroes and very valuable skills, very much worth the price.