How often do you replace carbon and phosphate media? Can you specific which type of media you use (seachem matrix carbon, phosban, phosguard, rwaphos, ... etc.)?
Both depend on you PO4 and DOC load. They could be used up in a day if high enough, or last a year if low enough. You can't tell what your demand is going to be off of some one else's tank, you just can't.
Running mine in a DIY reactor, which sits in the sump. I couldn't build two chambers in it, due to the height of the sump so I went w/one and stuck both in.
I mix some black diamond carbon with TLF phosphate media in a TLF phosphate reactor, changing it out once a month. I put the carbon on the bottom half, and gfo on the top half (to keep it from being 'pulverized')