I dont think I ever RMA'd a MSI product before, but most of my RMAs are around 21 business to process and get back to me, if I had to average it out. Some turn around in two weeks, some in a month.
All I ever cared about is making sure it was approved. I figure sometimes they are looking for the replacement, and considering sending something better if they dont have it.
You always hear stories of people getting back bigger drives, faster video cards, better boards, bigger sticks of ram, etc..
I usually note in the RMA up front if that works for me, by saying that a bigger drive is fine since it is not in a RAID array, full size ATX is fine if they dont have a Matx, etc....