According to the WHO 56 million people die each year. With the population at 120 billion, even though people are living older and the fact that medicine would have presumably improved, I don't think this would entail the mortality rate being proportionally lower per capita, as the state of being a lot older actually would nullify the improved medical treatment. So I'm going to assume a mortality rate in proportion to today's, that is (56 x 17) 952 million people a year.
Assuming reproductive rights were strictly suspended by the powers that be, and this injunction was adhered to by the citizens, or successfully enforced by the government(s), to drop down to a population of 8 billion from 120 billion would take 118 years. That is IF no extra person were born in that time.
After this period of 118 years, those who were babies at the start of the reproductive ban would be 118 years old, and eight billion humans would be between the ages of 118 and 150. The 118-year-olds would be the youngest generation.
If you assume the mortality rate per capita to be LOWER because of improved medicine, then this 118 year span would become longer. Even if you quadrupled the mortality rate, so that it were just under 4 billion people dying per year, it would still take, all other factors held constant, about 28 years to accomplish an 8 billion population goal.
This is assuming NO new births, and no voluntary or involuntary euthanasia.
The most destructive war in terms of lives was WW2, which claimed (higher estimates) around 80 million people. And in my opinion, a great portion of these were non-combatants dying from famine, disease and genocides.
So that's 80 million in a 6 year period, that's 13 million per year. Well it's a start. The deadliest earthquake on record killed about 800,000 Chinese. The Black Death of medieval Europe wiped out between a quarter to half of it's population (50 mil to 200 mil). This is a measly 10 to 40 million people a year. The Spanish flu of 1918 killed between 25 and 50 million people a year. So with extra mortalities thrown in we'd be getting there.
But don't forget this is with a complete ban on reproduction. Assuming a complete baby ban, and a mortality rate per capita four times what it is today, we'd be sitting around 28 years to achieve the goal. To reduce that time to 16 years about 900 million people extra would need to die per year. Likewise, you could assume a mortality rate of 8 times what it is today, per capita, and the natural number of attrition would drop the population down to 8 billion after 16 years, that's about 8 billion people dying per year.
Did I mention with a complete baby ban.