回複：How long does it usually take I-140 to be aproved for NIW?

Usually, if your NIW is in TSC, processing time is around 4 month. If in NSC, not sure, half year maybe.

If your case is strong, I don't think you need prepare anything now, at least if your case is in TSC. Or you need to prepare some more letters and documents for the REF since usually REF need answer within 3 month (if some delays, maybe you have only 2 month to prepare them). So you need think about which professor can write letters for you. Also you can collect more documents, such as citation, if you have time.