It's directly proportional to how often you use your brakes. Similar to how a Toyota Prius gets better fuel economy in city driving than highway, because city driving involves lower speeds (less drag), and it can recoup most of the lost energy from stop/go traffic with its regenerative braking. Regular cars just dissipate the energy needed to slow the car down as heat.

Similarly, if you ride a completely flat route where you rarely or never use the brakes to slow down, you won't gain anything from regenerative braking. If you're in a very hilly area or you're in stop/go traffic, you will get a significant return from regen braking.