Active Carbon layer thickness - DIY mini filter

I only have one plant in a small space - 1ft x1ft x 2 ft ,and I use a pc fan for exhaust. I grow in a mini tent ,hidden in a cupboard.
The plant hasn't even grown buds yet ,only a few pistils ,but the room gets filled with an "unorthodox" smell already.
I only expect it to get worse.
Strain is Short Stuff#1 ,and only 20 days old.So I decided to make a tiny carbon filter.
The exhaust fan is a 120 mm Antec 3speed ,like this one -- http://www.newegg.com/Product/Product.aspx?Item=N82E16835209005
The highest speed setting claims 79 CFM.

My question is ,what thickness (minimum) should the active carbon layer be ?,,,in order to be effective --given the fact that I can only set up the filter as the fan to blow through it?

I'm asking because I keep seeing contradictory discussions online ......on one hand they say that a pc fan can be used for a mini filter.........on the other they say that pc fans don't have enough power to circulate air efectively through active carbon.

UPDATE

Fan is pushing through about an inch of active carbon

I used to have it on the 2nd speed before the filter ,now it's on the 3rd and same temps so far. Noise is about the same ,the DIY filter is actually muffling the sound a bit.
It has been running for about 2 hours ,and I can say the smell is gone.
For my purposes ,it seems to do the job nicely.