I went to my my local shop yesterday to buy some more clear and hardner they recommend using pints or less because of what you mentioned. I have a bottle that has been opened for 1 month and i haven't noticed any probs. I am sure the experiened guys no more about this though.
It seems everytime someone mentions doing clear coat, it must be mentioned that it is the nastyiest little poison on earth (contains cyanide?), protective gear is a must!
Oz