I140 asking for me evidence

I submitted I140 under NIW to VSC by myself. My receipt date is June 5, 2002. I checked online today, a notice was mailed to me on March 3rd, 2004 asking for more evidence. But I have not received it yet. How long does it take to arrive? And I have to respond it within how many months? What kind of evidence do they generally ask for? Please help.

Thank you!