There is no maximum length. They can in theory be what ever length is agreed with the landlord at the start of the tenancy. However it is unusual to see anything over 12 months
As others have indicated, it's probably unusual for a 'fixed term' tenancy to be for longer than 12 months (but there's no reason why it shouldn't be). However there's no need for any specific term. The tenancy could be a 'contractual periodic' tenancy. See here:
http://www.communitie...ousing/pdf/138289.pdf