I tried to start a new thread for this, but for some reason it will let me reply to my post but I can't start a new post. What gives?

Anyway, I am thinking of using a DIY carbon filter. Like the one here:
http://boards.cannabis.com/grow-faqs...on-filter.html

My question is this...
I will be using it for the growbox and during drying/curing. How often do I need to change out the activated carbon? Should I just change it when I switch from the grow to the cure? Or do I need to change it a few times throughout the grow? Thanks.