MVM does not use data in NDP while performing ipV6 discovery.
You're right though, and the User Interface should give you a warning if you specify a large IP Range? I haven't checked myself, but the warning is to discourage folks from using it :-)
When the range is large the time estimate is roughly # of IP's X 0.12816 seconds. The User Interface will allow you specify (after warning you) up to 2^32 (4,294,967,296) addresses in one range... that could take up to 17 1/2 years!! Your MVM license might expire first?