Sometimes, there is just so much to say.  Paul knew he might not get the chance to be with these people again, so he wanted to give them as much instruction as possible.  Remember, there were no Bibles in those days (the New Testament hadn't been written yet).  How much can you absorb?  Do you hunger after God's Word? What would you have done had you been listening to Paul, who may have taught for 12 hours?  Would you have stayed?