You deserve a gold star if you check your brake fluid on a regular basis, because it plays such a vital role in stopping your car. However, if you have to regularly add fluid, it's likely there's a leak in your brake system that needs to be addressed, and pronto.
Ideally, the brake fluid reservoir (typically mounted near the firewall on the driver's side) should always be at or near the full mark. Brake fluid is part of a closed hydraulic system, and under normal circumstances the level in the reservoir should not change significantly.
As brake pads wear, though, the fluid level may go down a little. That's a sign that you need new brake pads. A soft or spongy pedal may also be caused by low brake fluid, which allows more air into the brake lines. The fluid provides the hydraulic force that causes the brake pads to be squeezed against discs or drums, stopping the car.
If the fluid level goes down frequently or drops a lot in a short time, that means you have a leak somewhere in the brake system, such as in the brake lines going to each wheel, the master cylinder or in the hardware at one or more wheels. If that's the case, you need to make an appointment with a repair shop.
Topping off the fluid reservoir every week or two is only a stopgap measure, and further investigation by a mechanic is required to find out why the fluid level keeps dropping.
