You need someone to do a smell test. I exhaust 24/7 also and have been using the same filter for over 4 years. I have a rheostat on my fan and run it as slow as possible. I check my attic for odors at harvest time and haven't smellled anything yet, nor has the wife.

And yes, there is a huge difference between grades of carbon. The best is from a mine in Austrailia. I believe it's the oldest deposit in the world, not cheap but certainly worth it.

How long your filter will last depends on the amount (and surface area) of carbon in your filter. It's also affected by the amount of moisture and heat passing through it.