There are two variables on the time required for quick migration:
- Your storage throughput
- How much RAM is assigned to the VM
Using a 4GB Fibre Channel SAN on 15K disks, a quick migration of a VM with 1GB RAM is taking me 9 seconds.
EDIT:
I just moved a VM with 28GB of RAM. It took exactly 70 seconds from start to finish. A machine with 4GB RAM took 12 seconds. Note that disk fail over always takes the same amount of time.