1: Cain Velasquez - 602 days
Former two-time heavyweight champ, Cain Velasquez, holds the dubious honour of longest lay off between title defences.
He first won the title on October 23rd 2010, winning by first-round TKO against Brock Lesnar. He would then lose it at the first defence in the first of what would become a trilogy against Junior Dos Santos.
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Velasquez would go on to win the title for a second time in December 2012 at UFC 155, again defeating Dos Santos. He would defend against Antonio Silva before facing and beating Dos Santos in a trilogy fight on October 19th 2013 at UFC 166.
We wouldn't see Cain again until June 13th 2015 when he recovered from a long injury lay off to face Fabricio Werdum, he lost the fight by guillotine in the third round.