If you did the RMA through the retailer contact them, they should sort you out with a similar alternative product, if not the same card.
No graphics card RMA should take no longer than 28 days, wherever you buy the card from, or whoever the manufacturer of the card is - OcUK are very good in this situation, and I'm not just saying that, do a search on the forums.
Some retailers - you can guess who, will try giving you a % credit note based on the age of the card, I never deal with those companies anymore, if someone tried that with me I would take them to the small claims court and get the full value back.
Asus are bad, 6+ weeks usually if not longer if you just leave it, as I said contact the retailer once 28 days is up and they should sort you out.
If you want the fastest RMA turnaround just buy Gigabyte, they are in the UK, EVGA and KFA2 would be a good bet too.
Not sure why MSI turnaround taking so long, the MSI Rep mentioned a while ago no longer than 14 days.
I'll have to do more research on current MSI RMA's, I may have to alter my list list here >>
http://forums.overclockers.co.uk/showthread.php?t=18231329