To change the default timeout after which grub boots into default Operating System goto /boot/gru/menu.lst.
search for key word timeout
you will see the portion of file as below.
change the time from 5 to 10 or any secs you need .
now grub waits for 10 secs before booting into default os.
## timeout sec
# Set a timeout, in SEC seconds, before automatically booting the
default entry
# (normally the first entry defined).
timeout 5