If you start a new seed, you will see the new content gradually over time because a lot of things require you to improve your character and get past certain bosses. But that is really an artificial stretching out of the new content. If you start from your old world like I have, in a few days you will have done the majority of the content. I have already built most of the new things you can build, crafted most of the new items you can craft, made most of the new food you can make.