The rule of thumb is to run twice as much water as the size of your container and then give nothing but water for the last two weeks. There is much debate as to whether or not this is the best method to be used. It may not be the best but it works.
I agree with Bob that it is considered that standard flushing method.
I use a flushing solution when I flush but not needed. There has been alot of debate about flushing in general and I have started only flushing for 5 days, next batch I am going to experiment with a single plant and not flush at all, look for the results in 9-10 weeks.