If you're talking about the default video player (called Gnome Videos (was Totem)), this feature is in the Wishlist category. It's marked In Progress after about 4 years, so there's no telling when it might actually arrive. It's not going to happen. According to Bastien Nocera [totem developer]:
I don't want to see options like that in Totem. FWIW, use Shift+Left Arrow if
you want to skip back by 5 seconds.
... I rejected a patch to make the arrow keys skip time configurable.
If you do wish to configure, you might want to look at alternatives such as VLC, SMPlayer, etc.
In VLC, the option is available, but hidden away. Go to the Tools menu and open Preferences. In the bottom-left corner, change Show Settings from Simple to All, and search for hotkeys
:

Set the jump lengths to the duration you want (in seconds).
You can also change the shortcuts for the jumps. (This option is also available in the Simple interface under the Hotkeys section - only the jump duration needs the advanced view.)