Seems to vary from place to place
When I went to the local INS office, I was told that it had to be 90 days since the ND for them to give me an interim. However, I got my EAD before the 90 day period ended. I understand it is a relatively painless procedure where you just take your 485 receipt notice and your passport and they issue you an interim EAD.
However I have seen posts here that suggest that you can get an interim EAD earlier. That was not the case in the INS office I went to.
Hope this helps.
Regards
seebeekay