Interesting discussion going on about PERM and RIR at
http://boards.immigrationportal.com/showthread.php?t=156852. RIR was initially implemented to expedite the GC process for IT-types (usually Indians) in the late 1990s. Then the rules were relaxed, I think around 2000, and everyone became eligible to apply under RIR. The only difference between RIR and regular process is the time it takes to get labor certification. In RIR, it usually doesn't take more than 6 months wheres as under regular process, it can take anywhere from 12-24 months.
Here is my understanding of the RIR [I am not a lawyer so plz take this as a guideline].
Advertisement: 1 month
RIR: 6 months
140: 2 months
The last stage can take 3-4 months if you choose consular process and between 12-24 months if opt for 485.