The manufacturers advice is a 9 day treatment of Panacur about 4 times a year, which is every 3 months. There are some questions about whether this is the best thing to do, but if you know your rabbit has E.C. or has been exposed to it I would go with this advice.
If you don't know if they have E.C. or not then vets will recommend different things. Personally I think it's a mistake to wait to see if they get ill, so I'm giving them Panacur between 2 and 4 times a year.