A good rule of thumb is to have your brakes flushed about every 30,000 miles (48,280 kilometers) or so. Note that brake flushing and bleeding the brakes are two different procedures. Brake flushing involves removing all the brake fluid from the system and getting all-new, clean fluid inside.
Also, how much does it cost to flush brake fluid?
A brake fluid change costs between $73 and $104 for the majority of vehicles. The cost of labor will make up the vast majority of the cost, with the brake fluid itself relatively inexpensive. The cost is largely the same no matter what make and model of car you drive as it's a fairly straightforward repair.
Why do we need to change brake fluid?
Water in the brake lines lowers the boiling point of the fluid, so stopping ability can diminish in hard stops as heat in the system increases. In addition, over time the moisture can cause internal corrosion in the brake lines, calipers, the master cylinder and other components.