6 weeks, it took the Simian Flu, just a month and a half to bring the world to its knees, 6 months on only the major world Governments were left hanging on by a thread. By now humanity had realized that the virus had a 1 in 500 survival rate. As the bonfire of human civilization began to gutter and die, the remaining Governments of the world attempted to hold it all together to act as the last bulwark against the night. In the end, they failed.
By the end of the first year, the final lights around the globe blinked out. If it had simply been the Virus that hounded humanity, the 0.002% dregs of humanity could have perhaps rebuilt in time, the fire of humanity greatly diminished but not snuffed out. But mother nature had long grown tired of our meddling so as we fell the Apes rose. By your estimates around 1.78M, humans would have survived the Simian Flu in the United States, however, they are 1.8M apes across the United States.
God only knows how many of those lucky human survivors remain after the collapse, but more concerningly how many of those Apes made it out of their enclosers how many more with there be in 1, 2, or 3 years? The world we knew is dead what remains is a planet now up for grabs, as two Sapient Species jockey for the top of the food chain. Only one can inherit this world and the other, Damned to extinction.