What version of ESXi are you using?
What is the VMFS version of this datastore?
What is your backend storage - local, SAN, NAS?
There are a variety of factors that can influence the time Converter will take. A resize will copy files over the network via SSL, but there are tweaks that can be done also. Best bet would be to use a TEST system to do a couple of these operations. That will give you the best idea to approximate from.