Reddwarf,
Everyone wants your money...
Follow what the manufacturer says in the owners manual. This sounds like a political answer. I've yet to see a premature failure due to lack of maintenance when the maintenance is performed according to this schedule. They are not always perfect, but they generally know what they are doing and have more testing experience than anyone else. Also, they face litigation regarding these issues, so they are careful and tend to overcompensate slightly. All the lube places want your repeat business more often so they push for quick intervals.
It all comes down to how long you want to keep your vehicle and what type of driving you do. If you drive in severe conditions, then follow what the owners manual says for severe conditions. If you drive in normal conditions where you don't have temperature and environmental extremes and most of your trips are more than 10 minutes, follow the intervals for normal conditions. If you want increase your chance to get to 300,000 miles which is 2 x the design life of the car, then half the maintenance intervals. This is not perfect, but essentially you change the fluids before they begin to degrade at all. With most maintenance schedules, a lot of assumptions are made about your driving habits. I do the 300,000 mile thing with my car, but my wife trades every 2 years, so I just follow the owners manual with hers.
I'm new to the AG crowd, but for what it is worth, I am a automotive quality engineer who is obsessed with this stuff. I also race motocross which allows me to test while having fun. You will find out what products are worth their salt and the ones that are not.
For brake fluid specifically, the time should matter over miles. Brake fluid is hydroscopic and absorbs moisture over time. If you are in a humid environment then it will probably need changed more often for good measure. My personal preference is to change it at the first hint of performance loss like a spongy feeling or bad stopping distance even with good pads on.
Its all about money; for you and the lube place. Automobile maintenance intervals from the manufacturer are generally set a bit conservative and then the lube places cut that down by more.
For the record, heavy equipment (Caterpillar, etc), which costs millions has set intervals for certain things, but the companies who really understand cost / benefit are performing their maintenance based based on fluid and part condition. Since the cost of such testing devices is not feasible for the car owner, and you really don't want to disassemble your car to measure wear, this type of condition-based maintenance is not recommended for cars. Some cars now have an engine oil life monitor which computes the actual driving conditions and correlates that to oil breakdown.
Remember that the average car owner turns the key and drives and does not consider maintenance until the car breaks down or a light comes on to remind them that the oil life is 0%.
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