It's a little subjective. If you don't do any map completion, jumping puzzles, or events and just move from story mission to story mission, you can do it in a few days. Hardcore speed runners can probably do it in less. Story chapters unlock every 10 character levels, ensuring you're at max level when you get to the end of the core game.

There aren't many achievements tied to the core game story. Most of the PvE achievements are related to things in the open world until expansions where achievements also exist in story instances. I've played since beta and haven't completed all of them. Achievements like "Shield Master" just take so much time, because you have to get 5,000 killing blows with a shield.

I hope this is informative.