Three to five days is an optimal amount of time to spend in Phoenix if you want to relax, get outside to hike or golf, and see the main sites like the Heard Museum and Scottsdale. Extra time will allow you to make some interesting side trips to nearby places like Tempe, Arcosanti, and Cave Creek and Carefree.
Remember that the Phoenix area is pretty sprawling and you'll probably find yourself driving fairly large distances from one place to another so planning ahead will help you save time and gas. If you're heading to the Heard museum downtown, for instance, you might want to visit the nearby Arizona Science Center and/or the Phoenix Art museum, too. If you're going to Taliesen West, do so before or after spending time in Scottsdale.
Until the city's new light-rail system is up and running (December 2008 at the earliest), Phoenix is best explored by car. Key attractions and activities occur in close-knit clusters, but the clusters are spread out over the Valley, making cabs cost-prohibitive. Rental cars are easy to pick up and return at Sky Harbor Airport; reserve before you go.
If having your own wheels isn't an option, many hotels offer free shuttle service to and from the airport, and to some guest destinations (usually within 5 mi of the hotel).
The Valley Metro bus system is a good alternative to walking from site to site in downtown and central Phoenix, but it gets less appealing in the peak of summer.
If you're driving to the Grand Canyon from Phoenix, allow at least two full days, with a minimum drive time of four hours each way. You can always anticipate slow-moving traffic on Interstate 17, but in the afternoon and evening on Friday and Sunday, lengthy standstills are almost guaranteed: something to remember if your plans involve getting back to Sky Harbor airport to catch a flight out.