If you are using Prime Video, there is no setting for adjusting the amount of time that the Auto Play feature provides.
Have you tried restarting your device to see if the issue persists?
To restart your device, disconnect the power cord from the back of the device or from the power outlet, then plug it back in. You can also use your remote to restart your device. Press and hold the Select and Play/Pause buttons at the same time, for five seconds. Or, select Settings > Device > Restart from the Fire TV menu.