With the advent of modern oils and additives, engine oil and coolant lasts longer than it ever has – but that doesn't mean you should ignore the service intervals.
It's generally recommended that you change your oil at least annually, or every 10k miles, whichever comes first. Although some manufacturers state shorter, or longer intervals between changes.
Coolant generally has a much longer service life, with some manufacturers recommending a 15-year, or 150k-mile change. Although a five-year or 50k-mile interval is a more common recommendation.
With engine oil and coolant, it's important that you consult your handbook, or Haynes manual to find out the correct intervals for your car.
Why you should check your fluid levels
Most of the fluids in your engine bay are critical for the car’s operation. Among the most important are the engine oil and coolant, which is why these should be checked weekly. A low engine oil level will cause the engine to wear more quickly, and if it drops to a dangerous level the engine could seize. A low coolant level could result in the engine overheating, which could cause severe engine damage too. Regular checking of the coolant level will alert you to a leak early, so any problem can be sorted before it leaves you stranded by the side of the road in a cloud of steam.
Brake, clutch and power steering fluid are still important fluids but don’t need to be checked as often. However, a drop in level from month to month could indicate a leak in the system, and it’s important to fix this as soon as possible, rather than to keep topping up the fluid. Bear in mind, however, that the brake fluid level drops as the brake pads wear, so don't be alarmed if it's not up the max-fill line.