How long does it take to get a replacement green card?

I was issued a resident allien card in NY Feb 1978, I understand card issued during this time had no expiry date on them( it was a blue and white card) I have lost this card and filled for a replacement card two weeks ago in the US. I have since returned to the UK to arrange shipment etc and are planning to return to the US within the next two weeks is this a problem ?