That's really hard to say. There have been times before when people thought we had discovered all there is to know, and then, poof... some other big question opens up. Just speaking as a graduate in physics, at the start of the 20th century, it looked like that had all been done, just a bit of tidying up to do, then Einstein had a think and boom, relativity. A little later, Max Planck started thinking about black-body radiation and the "ultraviolet catastrophe", uh oh, we can't explain that, but what if radiation is actually particles... boom, quantum mechanics.
Every answer just seems to ask even more questions. Where will it end? Nobody knows.