In a healthy brake hydraulic system, you should not “lose” fluid; it is a closed system. The only normal brake fluid “loss” you see at the reservoir is what’s effectively stored out in the calipers as the brake pads wear, but the total system volume remains the same. If the brake fluid reservoir is dropping faster than that, there is usually a leak or a component, such as the master cylinder or brake booster, allowing fluid to escape or be ingested.