Which Ubuntu ?
If you want to increase this time limit, you can edit the grub configuration file /boot/grub/menu.lst, increasing the seconds in the TIMEOUT part.
https://help.ubuntu.com/community/GrubHowto
If you have grub2 then you need to edit /etc/default/grub and change GRUB_TIMEOUT but read the instructions ...
https://help.ubuntu.com/community/Grub2
The default OS booted is defined by DEFAULT or GRUB_DEFAULT respectively and is usually 0 for the first entry in the grub menu, 1 for the second, 2 for the third etc.