Not immediately, since even the actual charge does not show up immediate just "pending". I don't know the exact timing but when the charge shows up, the credit shows up usually on the same day it was charged. For example, I had a charge on Dec 10 for $18.xx, right below it.. the credit showed up as -$15.00.

Make sure you are signed up for it on the card though. If you are signed up, you would know because you get an email every month with the subject "Earn your $15/month statement credit today" when the $15 credit is renewed. It has been great for me so far. I always ordered over $35 of stuff (so no extra fee) but some of the stuff is usually not available (or the shopper can't find it) so they just drop from the total original charge. So usually I ended up $25 or lowered with -$15 credit.