I've just been researching into this, given I've just seen Nobivac given wrongly. :roll:
The fact sheet says 'Duration of immunity: The duration of immunity was established as 6 months.'
http://www.msd-animal-health.co.uk/Products_Public/Nobivac_Myxo/090_Product_Datasheet.aspx
It also says
'In the present state of knowledge no firm recommendation for revaccination frequency can be made but it is suggested that animals should be revaccinated annually in May/June just prior to the myxomatosis season as most myxomatosis in Britain occurs in late summer, autumn and early winter months. Where there is high risk of myxomatosis infection revaccination every six months is suggested.'