Enough content to last one or two weeks minimum.
This is a very personal statement. And by that definition: Not one WoW expansion was worthy of your definition of expansion because a group of people beat most content of all those expansions within the first few weeks.
Let's talk hours of gameplay split into different activities like levelling, exploration, PVP, Raiding, mini games, etc.. than you have one way to measure amount of 'content'. But content itself is very hard to measure because in almost every game time spent on content is created by making the player repeat certain activities to achieve certain results. This is extra true for MMO's, but every game has that. And can you really count content once if it is designed to be repeated?
In that essence.. the original game had numerous hours of class stories, 8 in total, combined with 2 seperate stories for every planet as well as flashpoints, operations and all that jazz. But for a certain subgroup of players the content was done way too quickly because they choose to focus on 1 class story and 1 planet story, often not even experiencing everything along the way, then came to the repeated content of endgame and said there wasn't enough of that and complained the game doesn't have enough content, while effectively doing only 1/8th of the class content and 1/2 of the planet content. Meaning that, in total, they did probably only 20-30% of available content. But for them and their experience of the game, their complaint was valid. They had done what they wanted to do, even if it wasn't even half of what was available.
And this is no different in other MMO's or games really.
So you can't measure an expansion on content because content itself is much too personal and subjective.