Brake fluid is hydroscopic, which simply means it absorbs water from the atmosphere. As it absorbs more moisture it's boiling point reduces effectively reducing its ability to compress and apply the brakes. For those that don't understand how the hydraulic brake system works, if you apply 10 pounds of pressure to the brake pedal, the system multiplies that by at least 10, so your 10 lbs. of foot pressure applies 100 lbs. to the brake caliper/pads. Oversimplified, but you get it, I assume.
While it is a recommended procedure, there really is no firm maintenance interval for most manufacturers. For the record, most of the German makes use a different base oil and therefore recommend different intervals. As a general rule, the brake fluid gets darker (looks dirty) as it absorbs moisture as it is getting the moisture from the dirty air it absorbs. If your fluid looks to be a clear, light amber (Think Ginger ale) color, it has not absorbed much water. As it gets more caramel in color it is more saturated. Today's modern brake hydraulic systems are very well sealed from the atmosphere and therefore do not absorb a lot of moisture. The biggest source of moisture in today's systems is that which is created by hard, heavy braking that heats the fluid. It naturally creates condensation as it cools back down. Of course most of that condensation is what's already in the fluid, not something it creates.
With all that said, the procedure is something the average mechanically inclined person can perform on their own. I will provide a basic procedure here and can answer specific questions as they arise.
1. Buy 2 quarts of DOT approved brake fluid.
2. If you have access to a Mighty-Vac or similar hand vacuum pump, you can make a power bleeder by converting a large plastic container with a good sealing lid into a recovery tank. Using the vacuum pump hose as a guide, make 2 holes at least 1" apart in the lid that will fit the tubing tightly. Place a piece of tubing long enough to reach the container from the master cylinder with the container sitting level in the under hood area.
3. Put the tubing down into the fluid in the master and then put the tube/fitting from the vacuum pump into the other hole in the container lid. As you operate the vacuum pump, it creates a vacuum in the container which draws the brake fluid out of the master.
4. Once you've drawn as much fluid out of the master as you can, remove the tubing and fill the master to the top with new fluid. (Keep the fluid container closed tight when not pouring to minimize the amount of moisture it absorbs during this process.) (Also AVOID DRIPPING BRAKE FLUID ON PAINT!!! IT WILL EAT IT AND IT MAKES IT HARD TO REPAINT!!!)
5. Using the same tool as steps 2 & 3, you can go to each wheel and draw the fluid out of the bleeder screw. Starting at the right rear wheel (Working from the farthest point from the master makes it go faster so Rt rr; lt rr; rt ft; lt ft.) loosen the bleeder and attach the tubing to it.
6. Operate the vacuum pump to draw the fluid out of the system. If the fluid is noticeably darker than the new fluid, operate the pump until you see the new, lighter fluid coming into the container. (Clear tubing works best for this!) REMEMBER to keep the master full or you'll make more work for yourself!! Re-tighten the bleeder at each wheel BEFORE moving onto the next wheel.
7. Repeat step 6 at each wheel in the sequence in step 5, again, remember to keep the master full.
8. Adjust the fluid level in the master to the marked range on the master and close it up. You just flushed your brake system. Most auto parts stores will take/recycle the used fluid for you.
If this is too much, you can use an old turkey/kitchen baster with tubing attached to draw the master fluid out. Then following the steps above, get a helper to bleed the brakes using the brake pedal. One thing to remember when using the brake pedal method is DO NOT PUSH THE PEDAL ALL THE WAY TO THE FLOOR!!!! Use a light foot pressure to SLOWLY depress the pedal and stop before hitting the floor so the bleeder person can close the bleeder BEFORE the pedal is released.
If ANY of this seems too daunting or is not fully understood, I highly recommend you find someone who does understand or hire a professional!!
If you are local I have the equipment and knowledge to help you do this if you feel it's needed.