don't know about carbon as i don't run it, but for phosphate remover (rowaphos in my case) just change it when your phosphate levels are creeping up.
mine has been in for around 3-4 months now, and i'm still getting 0 on my tests.
reducing the input of phosphate as others have suggested by rinsing frozen food is good and by utilising a refugium with lots of algae to soak up excess phosphates will both help!
a good quality po4 remover won't leach the phosphates back into the water when it is full so there is no worry there. just test every week or 2 weeks (or in my case when i see a spurt in algae growth) and if the phosphate level is not 0 change your po4 remover.