I don't know about rescinds with Starwood specifically, but credits usually take a few weeks to hit.
If you're worried about the bill being due, call the 800 number on the back of your credit card and talk to a CSR. Tell them that you've rescinded and are waiting for a credit. They will issue a temporary credit which will tide you over until the next billing cycle. That way you can avoid any finance charges/late fees.
You can also do this online with a lot of cc companies (e.g. AMEX).
If you haven't received confirmation that you've rescinded, I'd do that as well.