Process Improvement Half-Life
The time it takes to achieve 50% of the target is called the process improvement half-life.
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Process Improvement Complexity
The actual improvement half-life is a function of the organizational or design complexity of your management systems. Processes with many steps, long cycle times, or that involve many people are complex and will require a lot more time.

For example:

GM’s order-to-delivery cycle was 54 days in 2000. GM cut their order-to-delivery cycle by 50% by 2004. There goal is an order-to-delivery cycle of 7-10 days. How many periods are needed to achieve the goal?

The formula is simple. We add the goal (7 days) to half the difference between where we are today and the goal. In GM’s case they cut their order-to-delivery cycle from 54 to 27 days or by 50% in four years. So, in their next cycle of four years they will be at 17.

–1st cycle: 54 – 27 = 27 in 4 years (2000 to 2004)

–2nd cycle: 7 + (27 – 7)/2 = 17

–3rd cycle: 7 + (17 – 7)/2 = 12

–4th cycle: 7 + (12 – 7)/2 = 9.5

–5th cycle: 7 + (9.5 – 7)/2 = 8.25

–6th cycle: 7 + (8.25 – 7)/2 = 7.625

–7th cycle: 7 + (7.625 – 7)/2 = 7.3125

To reach their goal of 7 to 10 days will take three more cycles or 12 years. We estimate their delivery cycle will go under 10 days in 2016. Of course GM’s real problem is that Toyota is at 12-15 days now, which means that Toyota has a 6-8 year learning curve lead on GM.

По-русски у меня не очень получается это сформулировать. Может, что-то типа "период времени, за который расстояние до цели сокращается вдвое"?

по аналогии с "периодом полураспада", "периодом полувыведения" и тому подобными терминами, которые тоже переводятся, как "half-life" и характеризуют процессы, математически описываемые похожими функциями.