name check 有有效期吗？is 15 months?

I-140 and I-485 turned in Aug. 2005. Checked last August, name check already been cleared, next month I-485 is going to be current, I remeber someone said name check is only valid for 15 month. Just worried about it, since I don't when it got cleared. Does name check only valid for 15 month? After that, need to do name check again?