Strolling through ploshchad Dvortsovaya and taking in the sights from the outside may only take an hour or so, but it's a great introduction to St. Petersburg. The entire Hermitage can't be seen in one day; you'll want to devote anywhere from a morning to two days to wander through it.
Touring the Square of the Arts section might take between two and three hours, unless you want to linger at the State Museum of Russian Art, in which case you might want to plan an entire day here. Outdoor attractions, such as the Summer Garden, are worth visiting at any time of the year. Note that the Neva is very close here, so it's windy year-round.