Normally, RFE issued after filing for I-485 is only to check if you are still working with current employer or not. However, there could be various other reasons as well. Before denial, USCIS issues NID (Notice of intent to Deny) as a last chance to fix your case instead of RFE.Therefore you do not really have to worry about having your petition rejected just as yet. But RFEs should be taken very seriously, you need to ensure that you have to rectify every ambiguity in your case.

USCIS never issued RFE in my case, but I had done enough research few years ago about case denial.Actually I had recieved my green card less than six weeks after I mailed the application for I-140/I-485. If you are on EB1/EB2 category, and if you provide enough verification for the RFE issued, and if the fingerprinting was already performed, you should get your green card within a month after you mail in the corrections.