Carbon removes DOCs (dissolved organic compounds) from the water. DOCs being present may results in an odor or water discoloration (think of driftwood leaching tannin and discoloring the water as a result). Depending on your water source and tank decorations, you may or may not have significant DOCs to start with. I don't and as a result don't normally run carbon in filters. If you have filter pads that automatically include carbon, it's not hurting anything, but not necessarily doing anything either.
There are different grades of carbon. My understanding is that grading is based upon porosity, which leads to available surface area. The grater the surface area, the better it can remove DOCs. Activated carbon is one of the better grades. Just something to keep in mind if you find carbon for really cheap somewhere (it might be of lower quality).
As for how much, it's really something you need to test and observe to get the right amount and timing down. Carbon doesn't trap DOCs indefinitely and once it's "used up" it should be replaced. I'd suggest not changing it once the discoloration and/or smell comes back to the tank. How often you need to change it would be based on how much you use at a time.
So I'm not sure anyone can confirm that 1lb per month for your particular tank is the proper amount or not. It'll be some trial and error on your part to figure out how much you really need.
