How long will it take a chlorine level to drop from 10 ppm to 3 ppm in a 7000 gal pool?

A few days. You will probably be able to use the pool when the level gets to about 5.0 ppm 1) you should know the difference between residual chlorine and Free chlorine 2) Chlorine lost depend on size of you pool and temperature 3) circulation of water , temperature & size of pool will increase chlorine lost