I would suggest everybody here not to stick to some certain intervals, but to listen to your body and flush whenever it tells you it needs it.
It seems like my body wants me to flush every two or three months. I do it and always get some results.
Intersting thing that my biggest flush was the one that followed my first stone free flush a couple of months later. I got huge stones out, the biggest I have ever passed in my 11 flushes, I still keep those in my freezer and always feel weird that I could pass something like that.