By default boot loading time out option in grub is few seconds in Ubuntu/Debian if we are using dual boot in our PC.
To increase this time you need to edit the grub config file.
Grub config file name is /boot/grub/grub.cfg
Open grub.cfg with your favorite editor and search for set timeout.
By default it will be set timeout=10 or set timeout=5
Just change this value as your wish, then restart your PC and check for your changes, that should work.