Another really great museum to enjoy and spend some time too is Tretiakov Gallery (Третьяковская галерея).
It is a huge collection of Russian paintings. Going through the piece of art is a good way to familiarize with russian culture and history.
The museum is located close to the Tretiakovskaia metro station on the orange line (Третьяковская) or Novokusnetskaia on the green line (Новокузнецкая). The area is also very nice to walk along with lots of nice bars and restaurants.
You should count at least 3 hours if you want to see the all museum and some more time if you want to enjoy more some particular pictures or artists work.
At the time of writting, the entry cost was 500 rubles.
My favorites being most certainly the ones from Vereshchagin (Верещагин):