FWIW, they say 6-8 weeks for the $100-back credit, but it actually only took ~1 week for me (to my pleasant surprise

) when I bought the PS3 back in late December.
Also, if you're in a rush to use it, you could ask them to get the card to you via next day delivery or something like that. Just explain your situation politely, and they'll probably be happy to help w/ that. That's what I did.
_Man_