Quitting time is when you can finally clock out and go home from work. In the cartoon the porcupine is thinking about putting in a little overtime. Another way of saying to work overtime is to put in overtime.
His coworker, the bear, tells him “suit yourself.” You say this expression to someone to mean “do whatever you want and what you think is best for you, but I don’t agree.”
The bear then warns the porcupine that if he works more hours to earn extra money, he’ll lose money because he’ll be bumped into higher tax bracket. If you get bumped into a higher tax bracket, you move into another range of income that is taxed at a higher rate.
Taken from : http://comics.com/