Running an apt-get update or an apt-get install occasionally times out on VMs that I am building before I have setup IPv6.  Instead of waiting the (seemingly) long time for it to timeout, just add:

-o Acquire::ForceIPv4=true

to the end of the apt-get command to force IPv4.

And to make it permanent:

create /etc/apt/apt.conf.d/99force-ipv4 and put in it:

Acquire::ForceIPv4 "true";