The primary principle of basic maintenance is to check the fluids. Also check the items in the Toyota maintenance schedule such as brake linings, air filter, belt tension, etc. Spark plugs go 60,000 miles for platinum and 90,000 miles for iridium, but fluids degrade gradually. As fluids degrade their ability to perform declines often creating subtle effects such as increased wear at seals or elevated operating temperatures prior to actual mechanical wear. If you're using synthetic lubricants, you have the benefit of increased film strength and improved resistance to thermal breakdown which minimize wear. Fluids you can inspect visually are power steering, brake and transmission as well as engine oil. (color, odor - has the fluid darkened, does it smell burned?) Probably the minimum period for changing most fluids would be 30,000 miles (power steering, transmission, brake), but you can change the fluid in the reservoirs yourself effecting a kind of continual replacement regimen. For service by a mechanic, you would just wait until the fluids darken and then pay to have them completely changed. Chances are, if you begin a conscientious maintenance program now, and stay with it, plus repair any minor faults as soon as they develop, you will tire of the vehicle long before it wears out, or can drive it virtually forever. I've seen reports of over 460,000 miles. Of course, the alternate philosophy is wait until something breaks to fix it and simply ignore fluid leaks until the levels drop quickly enough to affect safe continued operation. Toyotas don't last as long that way, but they still last a long time.