Whenever you feel like. It is good to do it every two weeks, but if you have pains and discomfort you can do it earlier.
About one month ago I had to reflush a week after my 12th flush, because I did not feel any better, even though I passed quite many stones. This was my first time I felt discomfort after flushing. So I reflushed and I feel perfect since then. I passed very big stones by evening after the last flush. So it was these big stones that caused the problems.