YouTube Hone by Amitai

YouTube Hone replaces YouTube's default left and right arrow seeking with a
user-configurable skip amount. The extension popup lets you choose the seek
interval, adjust the slider bounds and step size, and toggle the feature on or
off. Settings are saved locally, so the selected interval is available the next
time you open YouTube.

While watching a video, press P to enable or disable custom seeking, use
ArrowLeft and ArrowRight to move backward or forward by the configured
amount, and use Ctrl + arrow keys to tune the seek interval without opening
the popup.