There are too many variables to give you a fixed answer on this. Speed of your systems, networking, and congestion will have huge impacts on this. 256GB is a large VM, and I'd expect it to take some time. Our small (2-8GB) systems move in a few minutes, but our larger systems (64GB) take 5-10 minutes (we haven't moved one in a while, so I'm going from memory).
Colorado State University
We have 2 vm's running in that hypervisor with below specifications.
Max Mem - 200 Gb Max processors - 10
Mem - 130 Gb Processor - 3
Max Mem - 256 Gb Max processors - 12
Mem - 256 Gb Processor - 12
So i can say total memory used by these 2 vm's are 405 Gb and 15 processors allocated. I have another Hypervisor which have 546 GB free memory and only 6 processors free. In this case can i do a live migrate for both vm's to this hypervisor? When i do pre-checks which memory (Max Mem/Mem)and processor(Max processor/Processor) need to check for the same availability of free memory/processor in destination machine.