I have done a Storage VMotion with an VM with 1.16 TB in 2 hours an 6 minutes. I think cloning a VM will be very similar.
The VM is located on redundant Storage, so the data will be read once and written twice. So the whole size of moved date should be about 3,5 TB.
The Storage supports Hardware Acceleration so there is no traffic between the ESX-Host and the SAN. The SAN consists of 420 disks with 600 GB/15K.
The move was done while the VM was powered on and there was other traffic on the SAN as well. In this environment the VM moves with 153 MB/s and generates a SAN Load of about 460 MB/s. May be this can help you a little bit to calculate the estimation time.
Paul