What is the /etc/init/failsafe.conf file for?
One purpose for failsafe.conf is to prevents problems like this one with slow DHCP servers: https://bugs.launchpad.net/ubuntu/+source/ifupdown/+bug/838968
You shouldn't hack at the sleep timer. The problem is probably that you have an auto [ifname] line in your /etc/network/interfaces file for an interface that doesn't get an address. This frequently happens if you install using a wired connection, and then stop using that wired connection in favor of another one (e.g. wlan).