World-of-warcraft – access the garrison before I reach level 90

If we were to compare it to WotLK and you're inquiring about obvious effects of the surge, like queues, etc, I'd say a week to a month depending on the size of your realm. The more players on the realm, the longer the surge will last. However if you're referring to the crowding in the starter zones, e.g. Vashj'r or Mount Hyjal, that's already starting to dissipate as people spread out over them.

You cannot play the original World of Warcraft as Cataclysm changed the map, starting areas, and other things significantly.

To clarify, this does not require you to buy any of the expansions, but it does require you to patch your game so that you're on the current version of the game. Even without buying the expansions there are many changes that will effect how you experience the game.