The idea contained in these words, namely that the people always heap up their sins to the limit, is found elsewhere in Scripture. For example, in Genesis 15:16, God tells Abraham that it is only in the fourth generation that his descendants will possess the land of Canaan because the iniquity of the Amorites is not yet complete.
The idea is that a group of people—even a nation over a period of generations—may continue in rebellion against God for some time. God may withhold punishment for a long time, but all the while the people are heaping up their sins.
By continuing in their sin, they increase their guilt until the point is reached where God can no longer be patient with them and pours out his wrath in punishment. The scales tip, the cup overflows (see also Daniel 8:23; Matthew 23:32).1,2,3
16 by hindering us from speaking to the Gentiles that they might be saved so as always to fill up the measure of their sins. But wrath has come upon them at last!