Looks like it's 6 months in a 12 month period. https://www.gov.uk/importing-vehicles-into-the-uk/temporary-imports Seems a bit severe so I'm assuming there were aggravating factors - they'd probably previously issued a warning or some such
This is a Tweet From the NW Motorway Police
GMP NWMPG patrol has stopped a Hungarian vehicle that has been in the UK since August 2012. Vehicle has no tax, so vehicle seized