How can I reduce the amount of time before an OSX notification banner dismisses itself?
Open your terminal and run this command
defaults write com.apple.notificationcenterui bannerTime SECONDS
where SECONDS is a number for how many seconds you want the banner to be displayed.
Currently, you cannot change the duration. But you can dismiss it manually by placing the cursor over the banner and doing a two finger swipe from left to right on a trackpad (or a one finger swipe on a magic mouse).