You probably know that the Hermitage is not only one of the most famous museums in the world?, but it is also one of the largest.
The State Hermitage museum consists of 5 buildings (impressed?): the Winter Palace, Small Hermitage, Old Hermitage, the New Hermitage, and the Hermitage Theatre. These five buildings are called «main complex». When you visit the main complex, you go from one building to another without practically realizing it, since they are interconnected.
The museum contains over 3 million exhibits: paintings, sculptures, archaeological pieces, numismatics objects ect. If you spend 1 minute by each exhibit you would stay there for nearly six years. And this doesn't even include eating and sleeping time, not to mention that you'd cover the distance of about 24 km, walking from one exhibit to the other.
The famous collection of French Impressionist paintings is NOT in the Main Complex, it has been moved across the Palace Square to the General Staff Headquarters Building. You should plan some extra time for Impressionists collection, the tour there can't be squeezed in the Standard Hermitage tour.
The Standard tour is approximately 2,5-3 hrs inside and you will “cover” most interesting rooms and see the most interesting exhibits. Private guide speaking your language, guiding you at your pace and matching your interests is highly recommended in order you are not lost in rooms and arts and not miss anything.