This is a pretty large art museum. You may want to raise your standards a bit and be more choosey with what you want to stop and look at more in depth. If you don't, you could find yourself burned out on art. I was pretty casual with my visit, not stopping to glancing at every piece, and it still took me a good 2+ hours. And I still didn't even see everything (because my ticket didn't include all of it). In fact, if you're especially interested in something (like the temporary exhibit), I suggest you see it first.
They have some works by some pretty famous names, but exactly the top notch pieces in the world. However, my favorite was the Picasso collection which consists of a lot of cubist works that I really enjoyed.
The art scene in Prague is really underrated. Unfortunately, the entries for art museums on tripadvisor are kind of a mess. Do your own research if you are interested in seeing the art. But this place will probably be on your list.