People hate waiting. If you’re going to give a member of your community recognition for doing something good, you should give it now.
This goes for all rewards; money, fruit-baskets, thank-you e-mails, new opportunities, moderation powers, a call from the CEO etc…
The longer someone has to wait to be rewarded for something good they’ve done, the less important that reward will be. The less they will associate the behaviour with the deed. They might not do something good again.
Likewise, the rest of the community wont connect the reward with the member’s good deed. They wont immitate his efforts.
The sooner you reward others, the better they’ll reward you.