Most experts recommend washing your car every two weeks or so throughout the year as a general rule. If you live in an area with a lot of salt — either from a nearby ocean or from salt trucks on the winter roads — you probably should wash it more, as salt can corrode the metal and cause rust.
Those who spend a lot of time driving through the backcountry should give their car a little more TLC, as well. The aforementioned bird business is acidic enough to eat through your car's paint job if it's left too long. Same goes for dead bugs and tree sap, so they should be washed off as necessary.
Your car can go longer between washes depending if you don't drive it every day, or you keep it in a garage and out of the elements.
And while we might think of car washes as a summertime chore, you actually need to wash your car more often in the winter. Salt from the roads is one of the worst offenders, and it can cake onto the body of your car and wreak havoc. More frequent trips to the car wash, particularly those with hand-held pressure wands, will help protect your car from salt buildup.
Of course, the interior of your car is exposed to less salt and fewer dead bugs, but it still needs to be cleaned. Use a vacuum hose to get any particles (dog hair, dry cereal, bits of dried leaves) out of the seat crevices and off the floor mats. If you find any stains during the vacuuming process, you can use an upholstery spot cleaner to remove them. Dingy seats can be cleaned with a simple solution of warm water and mild detergent.
Leather seats, though, need to be cleaned and conditioned to keep the leather from cracking and aging, at least every three months or so. If you drive a lot (say for a living) or the interior of your car is exposed to the sun, condition the leather every two months.