Activated Charcoal is nothing more that really clean charcoal that has a
lot of 'active' sites that can all bonding.  When your gas vapors sneak into the canister
they will adhere themselves covanetly to the surface of the charcoal.  A small
organic molecule, like the junk in gas that can escape (ethanol, methanol,
propanol, acetone, diethyl ether, etc) will cling to the charcoal and proves very difficult
to remove.  But do we need to remove it? The surface area of activated charcoal is in the
neighborhood of 600 to 700 meters^2/gram ! (See Dunicz, B.; Surface Area
of Activated Charcoal by Langmuir Adsorption Isotherm, pg. 38,
357.  1961)  So what this says is that you can get a ton of junk on charcoal and that is
why we use it in these vapor canisters, water pumps, fish tanks, etc.  The lifetime for
you canister is dependent on how much vapor you are putting through it, but i'd probably
do what Dave said and sniff the stuff and see if you smell gas.

As for the thing blowing up... I don't think it's much of a worry.  I
teach a physical Chemistry laboratory and one of the experiments is to determine the
surface area of activated charcoal. After the lab I will rinse the charcoal and then shove
it in a 500 deg C oven to drive off all of the junk and hence, reactivate it. But I
would not do this in my oven at home in case it does want to blow up!

Matthew Pollard             http://www.uidaho.edu/~poll7356
Dept. of Chemistry               http://www.chem.uidaho.edu
University of Idaho                   http://www.uidaho.edu