IPv6 has widespread adoption in some countries (e.g. Japan, China, Korea) and some large organisations (an IPv6 workshop I did 6 years ago suggested IPv6 support was required for all networking gear sold to the US govt for example).
I think the short answer to your question is... "When a significant proportion of your target audience uses IPv6".
For a public internet site, the answer to that is likely "probably never". IPv6 was meant to solve the "we're running out of IPv4 addresses" problem. Since instead of IPv6, most organisations have gone down the network address translation route, greatly extending the life of the IPv4 address space.