A very long time, several things would make reaching such a number improbable
A) we would run out of oil long before that
B) Long before we reach such a number the world would have warmed so much that there would be no ice caps at all, raising sea level ~65m which would cover a very large number of cities
C) by the time we reach a number more like 700 there will be no more denying what is happening as by then there will be no Arctic sea ice in Summer and sea level will be around 1-2m higher causing us some major problems and financial issues, but by then deniers will be pretending they never said anything.
The current rate of rise is 2ppm per year but that is slowly increasing, if no action is taken that could grow within a few decade to more like 5ppm per year.
Probably never, as long as 70% of the worlds surface is covered with water. But if atmosheric CO2 levels reached 1% it would make many mammals a little more drowsy than normal and raise average global temperatures be a few degrees, but there would still be ice at the poles.
Now ratio of carbon dioxide pass 400 ppm in the atmosphere. The studies are going to reduce the level. 400 ppm is a large ratio. This much increase is after many years. But due to new revolution in the industries there is possibility to high this value again.
The unpredictable climate changes will be the result of large ppm of co2.
There will be direct effects on the growth, physiology, and chemistry of plants, independent of any effects on climate, because co2 is essential for the growth of plants.
Hopefully there is not enough oil, gas and coal in the ground for that. But if that did happen, Earth would be 10-20 degrees C warmer. If that sounds appealing to you, I suggest moving to a warmer climate. Anywhere less than 70 metres above sea level will be under water. Earth would still be able to support life. Even an atmospheric composition exactly like that of Venus would not bring Earth's temperature to 100 degrees C and wter boils at more than 100 degrees C under that much pressure.
What would the world be like then?
