After various trials, I settled into doing mine every 4 weeks. That seems to be the optimal amount of time for my body. I think a range of 2 weeks to 2 months in between until flushed 'clean' works. Less than 2 weeks is extremely hard on the body and doesn't give the older 'stones' much time to move forward. More than 2 months and new 'stones' can be forming making it more difficult (or at least taking longer) to get the really old ones out.
The most important thing in any area of health is to learn to listen to *your* body. If every 2 weeks is too tiring for you, your body is probably telling you to slow down a bit.