IMHO... 2 - 3 months....
I think that the filter is just getting established after only one month.... Also, I have been keeping the carbon seperate.... this way if I ever have a problem I dont have to breakdown multiple filters... 8 Fluval 405, 2 tanks, 125G & 110G, 4 filters each, only 2 have carbon... all carbon.
Lately, I have been experimenting with foam covers over the intakes.... in an attempt to keep the water going into the filters a littel cleaner so it should not need to cleaned as often... seems to working... I have cleaned the foam sleeves about 4 times and it is realy pre-filtering a lot of junk that would otherwise go right in to the filter...
Good Luck...!
mm