So the placenta should be fully functional as the fetus' sole source of nutrient, gas, and waste exchange by what timeframe? We would expect to see the placenta be fully functional by about 12 weeks post-conception. So our answer here is going to be 12 weeks.
From 6 to 8 weeks, we are still seeing some development of the placenta, growth of the chorionic villi, and development of those blood vessels within the chorionic villi, but by about 12 weeks, we should see a good maturation of that organ, to the point where it can act as the sole source of exchange between the mother and the conceptus. So our answer here is 12 weeks.