Timing on converting unmanaged disks vm to managed disks?

I really haven't been able to find much guidance on how long the process takes of converting a VM that uses unmanaged disks to managed disks. 

I have VM with just an OS disk that was able to be converted in about 20 min but I have some much larger VMs with multiple 1 TB secondary disks and I'm wondering how long those VMs will take... any guidance specific or general on this process?