The vast area of Yosemite and the Central Sieera  deserves as much exploration time as you can give it. Both geographically huge and filled with sights and hikes and attractions, this is the perfect region to slow down and take your time.
Though summer is traditionally the high season and spring is best for the waterfalls, Yosemite National Park  is gorgeous at any time of year. During the high summer season, however, traffic jams and parking problems plague the park, just as though it were a major city. Seriously consider parking at the visitors center in Yosemite Valley  and using the free shuttles to travel to different locations within the park.
Tioga Pass  is less congested, and parking more available, making it a viable summer option. Try to plan at least two or three days just in Yosemite Valley , with an excursion to Glacier Point . Adding Wawona , Tuolumne , or Hetch Hetchy ? Plan a week—though it will never be long enough!
Mammoth Lakes  makes a great ski weekend getaway, but prepare yourself for a long drive to get there. In winter, take a break from touring and spend your time on the slopes; in summer a week will cover you.
When planning a visit to Sequoia and Kings Canyon , you have a couple of options. The parks could almost be done as a day trip if all you want to do is follow the main road and maybe take a couple short hikes. To explore the parks in depth, including longer hikes, plan anywhere from 3–4 days to a week.