I always ran GFO and carbon 24/7. If you turn off the reactor, it can start to stagnate, which can be an issue. Changing once a month seems to be fine for steady state use of carbon and GFO, although GFO might need to be changed a lot more frequently when dealing with an initial high level of phosphate.
I'd definitely suggest this.. especially if you have any issue with phosphates-- the phosphates that were in my water were gone over night and the carbon is also great as well. It is a very easy, simple and efficient system to use. For the price, you can't beat the quality that your getting!