On Windows (Ethernet 4 is my network adapter):
netsh int ipv6 show subinterfaces "Ethernet 4"

   MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
  1500                1  777446697  121595362  Ethernet 4

Without my changes, when I was testing under Windows, I was unable to change the MTU to 1500 (it would change back to 1480 after a router advertisement).

On Linux:
root@Microknoppix:/# cat /proc/sys/net/ipv6/conf/eth0/mtu
1500