Hi George,
Howl already supports a quiet period. This is set in the Howl preference pane on the computer generating the notifications.
If you would like to turn notifications off for a while, you can go to your device Settings, tap the Notifications setting, and select Howl from the list. There you can turn off Sounds and Alerts temporarily. This feature isn't part of Howl, but how push notifications can be configured for any application.
Does this help, or is there more specific functionality that you are looking for?
Jon
Howl Developer