Change the GRUB Menu Timeout on Fedora
careful while changing the GRUB you can make the backup file first
Step 1. Backup the /etc/default/grub file.
cp -v /etc/default/grub /etc/default/grub.bak
Step 2. As the root user, open up the /etc/default/grub file in your favorite text editor:
gedit /etc/default/grub
Now, edit the "GRUB_TIMEOUT" line to set the time:
Step 3. In terminal, run the following command to generate settings from /etc/default/grub file.
grub2-mkconfig -o /boot/grub2/grub.cfg
Step 4. Reboot your system.