Score reporting to Game Center works almost immediately for me, even in sandbox mode.
Here are the few things you can try
- Make sure the the Leaderboard identifiers are correct when reporting scores (Should exactly match with "Leaderboard ID"s in iTunesConnect)
- Try Deleting the test data under "Manage Game Center" section of iTunesConnect
- Delete the application, launch "Game Center" application in your device and goto "Games" tab and remove your app. Reinstall the app and try reporting the score again.
- Make sure [gkScore reportScoreWithCompletionHandler:^(NSError *error) doesn't return any error