Timeout of grub boot

You have to edit grub in /etc/default (i.g. with nano: sudo nano /etc/default/grub)

not only modifying this line: GRUB_TIMEOUT=3

but also adding (after that) this one: GRUB_RECORDFAIL_TIMEOUT=$GRUB_TIMEOUT

Of course after these changes you have to do a

sudo update-grub

as explained here.