My answer in this question is 'I usually have my lazy days'. I used this many time already. But this is not what the interviewer only wants to hear. They also want to hear how do you cope up with ypur weak points.

Something like 'My weakpoints/weakness is/are blah blah blah, HOWEVER <insert how you cope up>'.

In my opinion this is the best way to tackle this question. One step backward, two step forward technique.