Going ultra low power with a reflective optical sensor
A human eye blink is almost always longer than 50 ms. If you're only counting eye blinks, you only need a few measurements during each blink—one measurement every 15 ms would be plenty. The datasheet for the OPR5005 shows typical rise and fall times of below 100 μs as easily achievable, so having the sensor on for 300 μs before each measurement should be plenty to allow it to settle.

Therefore, the sensor need only be on with a duty cycle of (300 μs / 15 ms) = 2%, which would reduce its average current consumption to (20 mA · 2%) = 400 μA.

If this was the only load on a CR2477 battery (capacity = 1 Ah), it would allow for a battery life of (1 Ah / 400 μA) = 104 days. In practice, other loads, such as the nRF52, will reduce this substantially.