At least twice a year.
After every flight a plane changes its oil
Yes, dealerships do oil changes. However, they often take longer than fast oil change places, and they are frequently more expensive as well.
Once a year or every 50 hours of use.
Change the oil/filter every 5,000 miles if you use conventional oil, and every 8,000 miles if you use synthetic.
Level should remain fairly constant. oil changes 3-5k mi. If you are losing oil, that is a sign of wear.
This website has the recomendations for all fluid changes. smg.toyotapartsandservice.com/index.php
If oil changes were kept up, never
First try individual oil change company websites (e.g. Oil Can Henry's), as they often post deals of their own. Coupon aggregator sites like retailmenot.com will post coupons whenever they are available. New cars often come with free oil changes. Some dealerships offer punch cards for a discount after a certain number of oil changes.
when the oil on the dipstick changes color from a clearish golden brown to brown or black then you need to change your oi.
Like with any vehicle, oil changes should be done periodically. Normally oil changes are recommended at about every three thousand miles, give or take. The color and viscosity of your oil can help you to make your decision. If it is black and sticky on the dipstick, it's time to change it out.
Lack of oil changes?
Under normal driving conditions you can easily go 7,000 miles between changes. If you drive mainly highway you can go 10,000 between changes. Also change the filter each time.