The Hetzner cloud IP is assigned instantly and i as able to assign the floating IP to the server itself or leave it for virtual servers (if one decide to setup virtualization like KVM, OpenVZ).

If want to assign to server itself, then i ran following (after modifying FLOATINGIPHERE in following command) on CentOS 6:
echo -e "BOOTPROTO=static\nDEVICE=eth0:1\nIPADDR=FLOATINGI PHERE\nPREFIX=32\nTYPE=Ethernet\nUSERCTL=no\nONBOO T=yes" > "/etc/sysconfig/network-scripts/ifcfg-eth0:1"
service network restart
Debian, Ubuntu.. tutorial is on Hetzner site: https://wiki.hetzner.de/index.php/Cloud_floating_IP_persistent/en