The brake fluid should be changed every year before the car is run for the season. Use either DOT 4 or DOT 5.1 and do not use silicone brake fluid.
While in storage, condensation will accumulate in the system. One feature of brake fluid is the absorption of water. When replacing the fluid, you are also removing the water and protecting the system. When water is allowed to stay in the system, it corrodes parts and lowers the boiling point of the fluid.
Silicone brake fluid doesn’t eat paint and that’s it’s only real feature. One of the down sides are it will not absorb water like normal brake fluid. You are never sure that all the water has been removed during flushing. Once agitated, it is very difficult to get the air out of the system. If the car previously had normal brake fluid and you are switching to silicone, any amount left of the previous fluid will have a chemical reaction leading to problems in the future.