A1. U can say u r looking 4 a more challenging and exciting working environment where u can excel and go up the ladder.
This way u can make ur interviewer feel that u r not looking 4 any mundane job. Also this will help u 2 better negotiate ur terms in case u r offered the job.
A2. I agree with 1 week though sometimes it mayt be bit longer 4 given reasons.

Number 1: you can always say that you have realized that the job that you are applying for is better for your talents and the change is more of a career thing.
Otherwise never say that your boss was an ass.
If you feel comfortable, you can mention that the salary was too little, this may get the guy to pay you more.

Number 2: If the "help" is wanted immed. they will call you within a week, otherwise they've found someone else, or its a scam