Honda claims the brake fluid in its cars should last four to five years "under normal circumstances," but driving in extreme weather or heavy traffic can affect this timeframe.
It's a good idea to flush the old brake fluid whenever the car requires brake servicing. Inspecting the condition of the brake fluid at every oil change is also good practice.
If the brake fluid in the reservoir is dirty or brownish or if the brakes start feeling ineffective, have the brake system checked and have the fluid replaced whenever possible.