Open up the /boot/grub/menu.lst file in your favorite text editor. you can use gedit:
sudo gedit /boot/grub/menu.lst
Now find the section that looks like this:
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the default entry
# (normally the first entry defined).
timeout 3
The timeout value is in seconds, specify the time you required. Save the file, and when you reboot you will have that many seconds to choose the menu item you want.
There are many other options avaliable in Grub -- check here
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