I really depends upon your anal-ness. How much of a maintenance nut you are. I am a maintenance nut, thus I keep my bikes very clean and operational to the Nth degree. On a road bike if it has hydro brakes, flush the system every year, especially if using DOT fluid as it collects moisture from use. Mineral oil systems tend not to do that, thus some mechanics say it is 'forever fluid", meaning mineral spirits don't need flushing for the life of the product. If it were mine, it would be flushed once a season. When I was service mech, I always recommended annual brake flush to the customer. Of course few did that, in fact only one person followed my advice, and his bike was something I would be proud to claim because it was well cared for.
Flushing the system gets the moisture and dirt out of the system. This is imperative for long term service life of the product. On my cars I flush the system every 3 years, motorcycle every year. I have a 40 year old vehicle still running with the stock brake calipers. Don't see that often as most will seize up long before the 40 year mark. Mama said cleanliness is next to godliness.