Once you've hit the balance, you'll have saturation equal to 1 + 4/5 + 16/25 + 64/125 ... It ends up summing to 5 (what a coincidence!). So 1 dose with a half

Once you've hit the balance, you'll have saturation equal to 1 + 4/5 + 16/25 + 64/125 ... It ends up summing to 5 (what a coincidence!). So 1 dose with a halflife of 5 years, applied yearly, hits equilibrium at 5x concentration.